What was the EASIEST gym ever?

Of recent ones and just generally - yeah - Skyla's pretty easy. Especially since you're probably sporting some freshly caught Electric types.

The first one I thought of when I saw the question though was that one time I caught an especially nice Jolly Scyther in Fire Red and had it on my team through the entire game and it absolutely demolished Erika's grass-type gym. That was the first time I actually felt sorry for a gym leader. That was more because of that awesome Scyther though...

My easiest was Falkner (GSC)

I liked the list from post 11, so here's mine.

Kanto

Brock: If you made the wise choice and didn't choose charmander then this was easy as hell. 4x weakness to grass and water lol.

Lt. Surge: Even if you chose squirtle, this was easy even without Diglett.

Blaine: Surf on a good pokemon = win. Same goes for the last gym.

Johto

Falkner: lv.9 lol

Jasmine: Steelix get OHKO'd by Feraligatr. Magnemite just dies.

Hoenn

1st gym: lol rock

3rd gym: Choose mudkip. Plz.

4th gym: Choose mudkip. Plz

6th gym: this is very forgettable and easy. Flying type is not the best, and the only good pokemon on that team is skarmory.

8th gym: It's the last gym. Why is there a luvdisk? It's ridiculous.

Sinnoh:

1st gym: why. Doesn't mattter what you choose. Easy.

7th gym: Yes having a million weaknesses sure helps in losing.

Brock was the easiest for me - always chose Squirtle as my starter and it made things pretty simple.

easiest leaders ever
4th gym kanto erika
1st gym johto falkner
2nd gym johto bugsy
6th gym johto jasmine
2nd gym Sinnoh gardenia
1st gym unova tro bros

Gen I (R/B): Erika
Gen II (G/S): Jasmine?
Gen III (R/S): Wallace/Juan
Gen IV (D/P): Roark
Gen V (B/W): Skyla

The Gym that I found easiest out of any of the games that I've played was in Oreburgh against Roark. Maybe it's because I over-trained my Prinplup going into the battle (one of the few times I've evolved my starter prior to the first Gym), but it seemed like I absolutely steamrolled his Pokemon, whereas in other first Gyms the Leaders put up a bit of a fight. Bugsy would probably be second on my list, just because I generally pick Typhlosion, which makes the battle awfully easy.

1st gen (Yellow): Misty, Pikachu use Thunderbolt! xD
2nd gen (Gold): I can't remember, maybe Bugsy?
3rd gen (Ruby & Emerald): Roxanne, Mudkip (well Marshtomp, I tend to overgrind xD) use Mud-slap! xD Use Water gun! xD xD
4th gen (Diamond and Pearl): Roark, Rock type normally tends to be easiest for me (Twiggy, Turtwig xD)
4th gen (Heartgold): Believe it or not, Whitney. Clefairy? Geodude, Rock Smash and it worked. Miltank? Geodude, Selfdestruct. Like 3 or 4 moves xD
5th gen (Black): Maybe Clay, because like Rock, Ground tends to be fairly easy for me.

Hello Falkner, how nice to meet you! Oh wait, sorry, got to go... just defeated you in 10 seconds flat. Kthxbai!

BW2: After grinding a Riolu to learn Force Palm, Cheren was nothing.

I don't think in my fourteen years of playing Pokemon that I have ever struggled to defeat Blaine. I think I've lost to all Gym Leaders at least once except him.

For me, I would have to say that the easiest was falkner.
They're simply such low levels that his pokes are a pushover no mater what starter you choose.

A pity really, I'd like a super challenging Johto.

Roxie (if you have a Magnemite), Burgh, Skyla, Brycen, Marlon, Falkner, Bugsy, Jazz, Pyrce, Candice, Byron, Koga/Janine, Blaine, Giovanni, Lt. Surge, Flannery, Brawley, Tate & Liza, Wallace, and every single Rock gym leader.