Spinarak is a common encounter on Routes 2, 30, 31, and 37 and on Headbutt trees in Gold and Crystal at Night only. It is also found on Route 36 and the National Park in Crystal. It cannot be caught in Silver.
You're probably thinking "Ew, early-game Bug-type! Ew, shallow movepool! Ew, it learns its best moves really late!" And for the most part, you are correct. Spinarak is often seen as a terrible Pokémon with low stats and a shallow movepool. However, if you take the time to really get to know the Spinarak line, you'll find that it's a real diamond in the rough, for as much as a Bug-type can be. It might not compare to the titans we know as Scizor, Pinsir, or Heracross, but honestly it's got quite a few things going for it, especially for those who chose Totodile (your rival's Meganium says hi). All in all, give it a chance. It might save your life.
Gym #1 - Falkner (Violet City, Flying-type): Sit this out. It's a Flying-type Gym and Spinarak is still rather weak anyway.
Gym #2 - Bugsy (Azalea Town, Bug-type): Poison Sting the Metapod to death. You will eventually win against Kakuna, but between its resistance to Poison and inability to be poisoned, it's going to be a long fight. Scyther is way too powerful; even Fury Cutter will do a number on Spinarak after a few hits.
Rival (Azalea Town): Taking out his Gastly and Zubat is a "no". Croconaw is probably too powerful and get it the hell away from Quilava. Bayleef, however, was meant to be fought using Spinarak, so Poison Sting spam and win this one!
Gym #3 - Whitney (Goldenrod City, Normal-type): Clefairy's Metronome shenanigans will likely pull something dangerous before you take it out, and Miltank will start a Rollout chain on you. Don't even try to Poison it; you'll just risk your Spinarak that way.
Rival (Burned Tower): Same things as before; leave it out. If you already have an Ariados, however, it gets a decent matchup against everything except Quilava, thanks to Night Shade.
Gym #4 - Morty (Ecruteak City, Ghost-type): At this point, you should have an Ariados, and actually, it isn't too bad here. Gastly and Haunter will probably go down to a few Night Shades as you take little damage from their attacks. Gengar is too powerful and will take more than a few Night Shades unless you overlevel, so avoid that.
Eusine (Cianwood City, Crystal only): Keep it away from Drowzee for hopefully obvious reasons. Haunter will probably kill itself with Curse so you'll be able to land the kill with Night Shade. Electrode has STAB Thunder, so steer clear of that.
Gym #5 - Chuck (Cianwood City, Fighting-type): You doubly resist Karate Chop and take a pittance from Fury Swipes, so Night Shade away unless it gets a streak with Leer or Rage. Poliwrath is really bulky, so it'll take a few Night Shades and Surf will hurt. Use something else.
Gym #6 - Jasmine (Olivine City, Steel-type): The Magnemites have pitiful HP, so Night Shade away. Steelix will ruin your day with Iron Tail, so stay away from that.
Rocket Executive battle #1 (Team Rocket HQ): Don't let Ariados fight any Normal-types here because they're immune to Night Shade. That, and be careful of the SelfDestructing Koffing. All in all, fight Zubat and leave it at that.
Rocket Executive battle #2 (Team Rocket HQ): Arbok is okay, Gloom is easy, but don't come anywhere close to the Murkrow, because of Pursuit. If you're playing Set mode, remember to switch Ariados out before Murkrow comes.
Gym #7 - Pryce (Mahogany Town, Ice-type): You can get away with Sludge Bombing the Seel and Dewgong. Piloswine has high HP (so no Night Shade) and resists Poison (so no Sludge Bomb), so keep it away from that.
Rocket Executive battle #3 (Goldenrod Radio Tower): Everything explodes and can't get OHKOed by Ariados. This is not a fight it should be fighting.
Rival (Goldenrod Underground): You can probably handle Sneasel, Magnemite (with either Night Shade or Dig, but preferably Dig), Haunter, and Meganium. Feraligatr's a bit too bulky and powerful and HOPEFULLY there's no need to explain why you shouldn't fight Typhlosion and Golbat.
Rocket Executive battle #4 (Goldenrod Radio Tower): Again, a fight that Ariados can manage, but the Murkrow is still there with Pursuit handy, so bail out before it gets to use it on a switching spider.
Rocket Executive battle #5 (Goldenrod Radio Tower): Sludge Bomb spam gets you far until they bring out the Koffing/Weezing. Also, stay away from any Houndour and especially that killer Houndoom. Yeah, not a battle for Ariados to be in.
Suicune (Bell Tower, Crystal only): Suicune is fairly strong, but Ariados can probably weaken it some with Sludge Bomb or Night Shade. Ariados is particularly helpful if you're trying to catch it, as Night Shade never crits.
Gym #8 - Clair (Blackthorn City, Dragon-type): You can probably take out the Dragonairs with Sludge Bomb, but Thunder Wave is annoying. Also, that Kingdra is stupidly bulky and powerful, but Ariados' STAB Sludge Bomb will be hurting almost as hard as its own Surf or Hyper Beam. Nevertheless, Ariados isn't the best resource to use here, and battling with it involves taking risks.
Rival (Victory Road): Again, take out Sneasel, Magneton, Haunter, and Meganium. Keep away from everything else.
Elite Four Will (Indigo Plateau, Psychic-type): STAB Psychic attacks will hurt, although you can probably handle Exeggutor with Sludge Bomb. Keep away from the rest.
Elite Four Koga (Indigo Plateau, Poison-type): You might be able to Dig Muk, but everything else has ways of beating Ariados: Venomoth has Psychic, Forretress is a Steel-type and can explode, Crobat's a Flying-type, and Ariados will live two hits and doom you with Spider Web + Baton Pass.
Elite Four Bruno (Indigo Plateau, Fighting-type): Double-resistance to Fighting means that you can handle everything here, but be very careful of Machamp because of Rock Slide. Ariados might live a Rock Slide, but it's dangerous. Avoid the Onix, too.
Elite Four Karen (Indigo Plateau, Dark-type): You can easily handle Vileplume and Gengar, just watch out for Destiny Bond as you will definitely be outsped. Umbreon's really bulky, but if you can poison it it'll go down fast. Murkrow doesn't have any Flying moves and is 2HKO'd by Sludge Bomb, so go ahead. Keep it AWAY from Houndoom.
Champion Lance (Indigo Plateau, Flying-type): Gyarados' Rain-boosted Surf or Hyper Beam will hurt Ariados a lot while Gyarados will likely live 2 hits from Sludge Bomb. Charizard and Aerodactyl shouldn't need explaining and one Dragonite, specifically the ace, has Fire Blast. Even the other two Dragonites are probably not a good matchup for the spider anyway, so keep it out.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
Gym #9 - Brock (Pewter City, Rock-type): If you already have Giga Drain, the Rock/Ground-types here will be a cakewalk. If not, Ariados needs to sit this out. Omastar and Kabutops are not feasible either, unless you're certain to outspeed them, which is hard for Ariados.
Rival (Mt. Moon, optional): Sneasel and Meganium are still perfectly doable. Magneton becomes more dangerous, but should still be an okay fight so long as you keep watch on Ariados' HP. Gengar is evolved now, and a lot more annoying, but you can attempt it with Night Shade. Avoid the rest.
Gym #10 - Misty (Cerulean City, Water-type): Interestingly, Golduck has Psychic and Starmie does not. Of course, you need to avoid Golduck. Giga Drain is great against Quagsire, while the rest takes more damage from Sludge Bomb. Avoiding Starmie is still a good idea, as its Special Attack is quite scary for Ariados.
Gym #11 - Lt. Surge (Vermilion City, Electric-type): Everything here is fast, and the Electrode can even explode. You can take on Raichu and perhaps Electabuzz, but should keep Ariados away from the rest.
Gym #12 - Erika (Celadon City, Grass-type): Perfect gym for Sludge Bomb spam!
Gym #13 - Janine (Fuchsia City, Poison-type): Everything here is at very low levels, to the point Ariados shouldn't be scared at all. If you have Return, you can make short work of everything, though you still should have better alternatives for Crobat and possibly Weezing.
Gym #14 - Sabrina (Saffron City, Psychic-type): The typing of this gym screams "no" all over. Unless you have Hidden Power Bug to take on Mr. Mime with, it's not worth it. Espeon and Alakazam WILL outspeed and kill you.
Gym #15 - Blaine (Seafoam Islands, Fire-type): Again, bad matchup all around for Ariados. It'll get fried before it can even act, so don't even think about doing this.
Gym #16 - Blue (Viridian City): Ariados can do well against Exeggutor, thanks to it not having Psychic STAB. Rhydon can hypothetically be OHKOed by Giga Drain, but it depends on your level, and it will retaliate with a very strong Earthquake if you don't make it, so beware. Everything else is either too strong or has supereffective moves, so don't bother.
Rival (Indigo Plateau, optional): Again, Sneasel and Meganium are Ariados' bread and butter. The rest, sans perhaps Gengar, is largely out of its league and requires team support to defeat.
Red (Mt. Silver): Ariados easily handles Pikachu with Dig or Sludge Bomb. You can probably handle Venusaur using Psychic or Sludge Bomb. Charizard and Espeon are instant "nope!"s and Blastoise is probably too powerful. Snorlax is super bulky and can paralyze you, but if you can take a hit and poison it, it'll go down much, much faster. Just bear in mind Ariados is not that bulky, and Snorlax is strong.
Just like literally every other Bug-type in GSC, Spinarak and Ariados don't have much in the way of moves. When you get it, you have Poison Sting and String Shot. Then you get Scary Face quickly at level 6. Five levels later you get Constrict, which is very easily the worst attacking move in the game. If you can put up with it for six more levels you are rewarded with a saving grace: Night Shade. This is Spinarak's best way of dealing direct damage for a long time. Then comes evolution! And after that is Leech Life at level 25, which isn't great, but hey, it's better than Constrict. Up next is Fury Swipes at level 34, which is kind of bad, but again, it's not like you have anything better. Level 43 gives you Spider Web, which you hardly need because opponents almost never switch out in this game (although it does let you do fun stuff like catch Abra without it Teleporting but that's a story for another day). At level 53 (by that time you should be in Kanto) you get either Screech (GS) or Agility (Crystal). Both are great moves: Screech allows Ariados to do a ton of damage; Agility allows Ariados to sweep. The final move is Psychic, which won't do a ton off its 60 Special Attack, but it's probably better than most things you have.
TM moves! There's not a lot worth mentioning, unfortunately. Curse is always viable, especially in tandem with Agility, but remember that Ariados is usually better off attacking right off the bat rather than taking a few turns to set up. Hidden Power is great if you get Bug. If you do, that instantly warrants a moveslot. Giga Drain is a spectacular move for Ariados because it makes grinding against those pesky Gravelers much easier and also gives Ariados a better shot against stuff like Brock and Blue's Rhydon. Return is nice for power. Dig is a great move on Ariados because it's your best shot against Steel-types, who wall Ariados' other moves (besides Night Shade) for days. Psychic may sound nice, but remember that Ariados learns it leveling up. Finally, there's Sludge Bomb. If you value the life you've been given then TEACH ARIADOS SLUDGE BOMB. IT WILL MAKE IT THAT MUCH MORE VIABLE. STAB SLUDGE BOMB OFF OF 90 ATTACK HURTS.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Spinarak Line Ratings
At what point in the game should I be evolved? Right before fighting Morty is best.
How good is the Spinarak line in a Nuzlocke? While often overlooked, Ariados is actually a good enough Pokémon in the hands of a good trainer. While it needs babying before evolution and Night Shade is its best attacking move for a long time, once it learns Sludge Bomb it's a monster. It's pretty bulky, it's very powerful, and it may be slow, but in Crystal it gets Agility to resolve that. If you can put up with it being dead weight for a while it'll serve you well.
Weaknesses: Rock, Fire, Psychic, Flying
Resistances: Fighting, Bug, Poison, Grass
Neutralities: Ghost, Normal, Dark, Dragon, Electric, Ice, Ground, Steel, Water