1st Gen How Would You Rank The Difficulty of Kanto Gym Leaders?

1. Sabrina - Considering the fact that Psychic types were broken beyond repair in Gen 1...

2. Surge - All of his Pokemon are pretty fast, and Raichu is especially dangerous with Thunderbolt.

3. Koga - Painfully bulky. All of them. But luckily, they dont have much attacking power.

4. Giovanni - Although he has a team of five fully evolved (at the time) Pokemon of one of the most offensively powerful types, they're all weak to water, and they're all weak to ice.

5. Erika - Actually a good setup, with a seemingly strong lead in a fully evolved Pokemon. Sucks that Grass is a bad type, especially with the amount of choices you have before this battle. Also, Tangela's moveset.

6. Misty - Only two Pokemon, and they're both weak to a pretty common type in Grass. Nice how this game works, isn't it?

7. Blaine - Ember in the seventh gym battle? Two LC Pokemon. In the seventh gym battle? Bye.

8. Brock - Starters.

1) Sabrina
2) Koga
3) Erika
4) Surge
5) Giovanni
6) Blaine
7) Misty
8) Brock

Sabrina is the hardest, no question.
Koga and Erika often gave me some trouble because they have a few status-changing moves.
I never had too much trouble with Surge.
Giovanni and Blaine are easy, at that point of the game you should already have a water pokémon able to wipe out their teams.
Misty and Brock could be a bit higher if your starter is Charmander (or Pikachu for Brock in Yellow), but even then it's not too hard.

1) Sabrina - Psychic OP
2) Koga - Bulky and can be annoying
3) Erika - Can get in Status / Wrap hell, which is specially bad considering how they work in Gen I
4) Blaine - Fire Spin hell

5) Giovanni - Rhydon could be troublesome given Fissure and Horn Drill ... but you'd need to be outsped by Rhydon, which
isn't going to happen anytime soon
6) Surge - You get Dugtrio and Dig before him
7) Misty - Can be annoying if you picked Charmander
8) Brock - Can be annoying if you picked Charmander, but easier

1) Misty - The one I had most trouble with. STAB Bubblebeam + a super fast and strong special attacking water type was tough that early on!
2) Brock - Tough to handle without super effective moves, and Bind can be pretty annoying.
3) Sabrina - Fast and strong!
4) Erika - Status can be annoying, especially if you have neutral attackers.
5) Koga
6) Lt. Surge
7) Giovanni
8) Blaine

It was hard to rank most of them since I find the second half to be all a similar level of easy, but Brock and Misty gave me the most trouble personally.

Brock: Ya gotta admit, even for Charmander + Pikachu as starters, you could always train a Butterfree or a Mankey (Yellow) to make life easier. Low difficulty.
Misty: Charmander'll be your main problem for R+B, but there's Viridian Forest's Pikachu + Oddish/Bellsprout N of Cerulean. Low-medium difficulty.
Lt Surge: I can see Bulbasaur being your main dude here. Otherwise, train a Diglett. Its intense speed + Dig move are quite intense for him. Medium difficulty.
Erika: You can teach Squirtle Ice Beam + Bulbasaur can tank her team. Pikachu is unlucky here. Low-medium difficulty.
Koga: Weezing has Explosion. If you don't have a tanky Venusaur w/ Leech Seed, Dugtrio w/ Dig or a really tough Hypno, then train. Hard difficulty.
(For the Venonats/moths, just send out Fire, Flying or Psychic.)
Sabrina: Psychics! Psychics everywhere! Train a lot! And always save before fighting her!!! Really hard difficulty.
Blaine: Not so tough, even with Venusaur or Pikachu. Just rely on the rest of your team. Nuff said. Easy-medium difficulty.
Giovanni: Except for Yellow's Persian, he's easy to deal with. Grass, Water + Ice deal with it easily. Easy-medium difficulty.

Hardest to easiest for gen 1 Kanto:

Sabrina: Gen 1 Psychic types are broken, nuff said. Alakazam is crazy in any case.

Giovanni: Mainly because of the high levels and Persian in Yellow. Also has some OHKO moves that can suck.

Misty: There's a good chance you won't have an electric or grass-type this early on. Starmie is also an insane challenge at level 18, reminds me of Whitney's Miltank

Brock: Depending on your team, can be an absolute wall. Pretty tough for a first gym leader of any Pokemon game.

Blaine: Fire types with some decent coverage moves, but nothing a rock type can't take. Arcanine is tough though.

Lt Surge: Easily walled by a ground type and no real way around it, but without it he can pack a punch with T-Bolt.

Erika: Flying and fire destroy here and you almost certainly have one of them by now, but she can mess you up a bit with powders and crit Razor Leaf. If you have Beedrill it also trivializes this.

Koga: Easiest as his poison-types aren't very good. Especially in Yellow with just Venomoths I think. High-level, but bad. No movepool variety either