Rate the difficulty of each gym leader

[Ether's Bane]

In my opinion:

RBY:

No recollection

GSC:

Falkner: 5/10 (Not particularly easy or hard)
Bugsy: 4/10 (Scyther may be a problem, but a Geodude/Onix/Quilava will pack it off quickly)
Whitney: 2/10 (Get a Machop. 'Nuff said.)
Morty: 2/10 (Farfetch'd/Hoothoot/Noctowl/Pidgeotto. Shadow Ball is futile.)
Chuck: 7/10 (Poliwrath is absolutely brutal if you can't contain it)
Jasmine: 6/10 (Steelix is a bit of a problem)
Pryce: 6/10 (Even with Red Gyarados, Piloswine is no pushover)
Clair: 8/10 (Would be a 10/10 if Blackthorn City wasn't immediately preceded by the Ice Cave)

(No memory of the Kanto GLs)

RSE:

Roxanne: 4/10 (Nothing particularly noteworthy)
Brawly: 7/10 (R) 0/10 (S/E) (You pretty much need a Beautifly/Dustox/Ralts in Ruby. In Sapphire and Emerald, just use Sableye)
Wattson: 3/10 (R/S) (if Mudkip) 5/10 (R/S) (non-Mudkip) 7/10 (E) (Marshtomp makes it easy. Shroomish's Leech Seed and Makuhita's Fighting moves are good alternatives, but Manectric is tough in Emerald)
Flannery: 10/10 (R) 8/10 (S) 9/10 (E) (Brutal without Sableye, even with a water Pokemon. With Sableye, still brutal, but slightly less so)
Norman: 10/10 (R/S) 8/10 (E) (Two Slakings. However, he's easier in Emerald, with a generally weaker team)
Winona: 8/10 (Altaria will be hard if you don't have Castform)
Liza & Tate: 6/10 (R/S) 8/10 (E) (Lunatone and Solrock are moderately hard; the addition of Claydol and Xatu is a real problem)
Wallace/Juan: 7/10 (Both are fairly hard, but nothing ridiculous

FRLG:

Brock: 2/10 (Mankey/Squirtle/Bulbasaur. Yeah.)
Misty: 6/10 (Starmie is a pain, even with Grass-types/Pikachu, but not over the top)
Lt. Surge: 1/10 (Diglett's Cave is a short hop away. Yeah...)
Erika: 3/10 (A myriad of Fire- and Flying-types nearby; watch out for Vileplume, though)
Koga: 8/10 (Muk and its Minimize are insane)
Sabrina: 7/10 (Alakazam can wreak havoc if left unchecked)
Blaine: 6/10 (Moderately hard all around)
Giovanni: 6/10 (^)

DPPt:

Roark: 3/10 (Generally rather easy)
Gardenia: 3/10 (Roserade is only a problem if you have no Flying-types)
Maylene: 10/10 (D/P) 8/10 (Pt) (In D/P, absolutely insane. In Platinum, less so, but still hard)
Wake: 8/10 (Gyarados and Floatzel can really damage your team)
Fantina: 9/10 (D/P) 6/10 (Pt) (Mismagius is ridiculous in D/P, but with a watered-down team, in Platinum, she's not as hard)
Byron: 2/10 (Undoubtedly the easiest Sinnoh gym leader)
Candice: 8/10 (D/P) 9/10 (Pt) (Already hard in D/P, but Froslass is added in Platinum)
Volkner: 5/10 (Not overly hard, but make sure Ambipom doesn't set up for him to sweep)

HGSS:

I don't own either

Descriptions to come soon.

 

[1. Luftballon]

am I to be led to believe that you felt, in GSC, whitney was easy?

also, ratings are fairly useless unless conditions are specified. so maybe you shouldn't have posted until you wrote the descriptions.

 

[ultraviolet]

I've always found Wattson to be really hard if you pick treecko and don't like using geodude. :|

 

[Autumn]

am I to be led to believe that you felt, in GSC, whitney was easy?

hey, to some, she /was/ easy. I never understood all the frustration with her, personally.

 

[1. Luftballon]

she's an ai trainer with a flinching move.

in any case, it's not particularly that whitney was easy; I find it difficult to believe that OP finds falkner and bugsy harder.

 

[Mustardear]

I'm not sure how anyone can find Giovanni hard, with access to Surf and an abundance of Water-types. I don't think I've ever had trouble with him whereas I have had trouble with Whitney (if Machop, which you can only get in an in-game trade, is the only legitimate answer to her then it's hard to say that she was an easy gym leader...)

 

[Lord Shyguy]

Most of the Hoenn gym leaders (in Emerald) gave me a bunch of trouble. Everything always worked out for them... Morty (in GSC) was hard too, only because I didn't have a normal type and he was spamming the Mean Look/Curse combo.

 

[Michi]

I'm only doing two regions for now. :P

GSC -
Falkner - 4/10 - Easy with Cyndaquil or Totodile. Chikorita does horribly. However, you could always just trade for Rocky, who made the entire battle a piece of cake.
Bugsy - 3/10 - I love Quilava, so I always murdered Bugsy. :P On playthroughs with either other starter, Normal attacks usually wiped him out.
Whitney - 9/10 - You need to consider that many people did not know that Machop was available by the time you reach Whitney. I believe the only one you could get at that point was a trade in the Goldenrod Mart for a Drowzee.
Personally, she was hell for me - I only ever liked to choose Cyndaquil, which obviously didn't work well against her. >_< I was one of the people who never oes around talking to every stranger to find trades, so I was quite oblivious to the existance of Machop. The only reason that she isn't a 10/10 is that Haunter works great against her, if you had one.
Morty - 7/10 - Morty was also a challenge. I could usually make it to Gengar easily, but With only Quilava and one or two more Pokemon left. Gengar usually defeated me if Quilava was less than level 30. I eventually learned to win by training a Ghastly from Sprout Tower.
Chuck - 6/10 - With a Typhlosion, he was sometimes difficult to defeat with his Poliwrath. However, I usually ended up winning if Typhlosion could hold out, or with another pokemon after Poliwrath was weakened. With Feraligatr, it was much easier and I would nearly always win. With Meganium, I would rip him to shreds. :D
Jasmine - 6/10 - Flame Wheel or Surf. :P Generally swept her team, for me. With meganium, however, she was much tougher. I had to trust my Tentacruels and Onixes.
Pryce - 3/10 - Typhlosion murdered his gym, Feraligatr was okay, and Meganium did well with Body Slam and other normsl attacks.
Claire - 8/10 - Typhlosion usually got murdered on the Dragonair, but sometimes made it to Kingdra, where he was ruthlessly obliterated. XP Feraligatr could sometimes hold out through the entire battle, but not always. Meganium did horribly at this gym, for some reason.

RSE
Roxanne - 1/10 - Mudkip and Treecko annihilate her, and with Torchic all you have to do is evolve it into Combusken.
Brawly - 3/10 - His Makuhita was oddly sturdy, but could be easily defeated with Wingull or Taillow.
Watson - 3/10 - Difficult with Grovyle, but the other two easily murdered his team.
Flannery - 4/10 - Easy with Swampert, but the other two never seemed capable of sweeping her team alone. I always had to use Pelliper or another water type.
Norman - 9/10 - I must have trouble with Normal type Leaders. This guy was insane! The Slakings usualy took out nearly my entire team, and then as the died I'd see that stupid Vigoroth. The only pokemon I could beat Norman with easily were Swampert and Makuhita/Medicham put together.
Winona - 4/10 - She was never much trouble for me, unless I had Sceptile. If I did, my other Pokemon (most of the time Pikachu) could deal with her.
Liza & Tate - 10/10 - These guys were the hardest for me. I'd always have a level 44 Wailord and a level 46 Blaziken/Sceptile and always lose horribly until I got lucky with a critical hit or two. With Wailord and Swampert, I still couldn't win very often - with any combination, one of my pokemon would be down in a single turn and the other would follow by the fourth or, most of the time, the third.
Wallace/Juan - 0/10 - With Wallace, the starter and Kyogre/Groudon swept him. Rayquaza absolutely murdered Juan. They made terrible mistake by letting you get Rayquaza so early in Emerald.

That's all for now!

 

[nothing to see here]

I can't really remember most of them well enough to make a list like this off the top of my head, but...

Maylene: 10/10 (D/P) 8/10 (Pt) (In D/P, absolutely insane. In Platinum, less so, but still hard)

...really? I've never heard of anyone having a hard time with Maylene. If I remember right, she was easiest Gym Leader for me in Platinum (except maybe Roark?), and my little sister beat her without any trouble in Pearl, too. She was actually amazed at how unexpectedly easy the fight was...

 

[Enkoe]

Gym Leaders:

HGSS
Falkner; 2/10; very easy. If I had Chikorita, I'd use a Pidgey for this battle; I recently won with my trusty Wing. Quilava swept his Pokemon as if they were ants. Haven't tested Totodile (possibly never will). Never used Rocky in this battle. With my Furret-solo runthrough, he was a piece of cake.
Falkner's "illegal" Pidgeotto was obtained from Viridian Forest. They appear there at Level 9 at least 1% of the time.

Bugsy; 3/10; also easy. If I had Bayleef, I'd murder Bugsy with Wing. Quilava also murdered this guy. On Furret-solo, I murder his coccoons, and took a little difficulty with Scyther.

Whitney; 7/10; HARD. F*** you, Whitney. Miltank murdered Quilava and Furret; haven't gotten to her on my Chikorita run. D: My strategy with Quilava was Ember until she burnt, when she healed, I would keep using it until Miltank burnt, then continously attack with Flame Wheel and such, healing when nessecary. Clefairy was defeated by a critical Flame Wheel, yay.

Morty; 5/10; easy. Quilava would simply murder his ghosts. On my Furret-solo, I made her use Foresight and murder the ghosts.

Chuck; 8/10; OH MY GOD. YOU ARE HARDER THAN WHITNEY. F*** YOU. I remember using Quilava and Kenya the Fearow to take him down. Furret survived only 2 seconds.

Jasmine; 10/10; YOU ARE A F***ING BITCH. Never took her down. Furret never made it past Chuck. Quilava murdered in 2 seconds.

Pryce/Clair: never beaten.

DPPT (Platinum only)
Roark; 1/10; I always chose Piplup. This guy is a joke.

Gardenia; 5/10; slight trouble. I used my Staravia. Yay for a slightly -far- Pluck giveaway. :D

Fatina; 5/10; also slight trouble. Didn't used ghosts; I recall murdering Mismagius with a Luxio. Staravia is awesome here.

Maylene; 6/10; slightly more troubling. I recall only using a Luxray.

Crasher Wake; whenever I got to Pastoria, I got bored and restarted. Hm.