Pokemon games have always been easy, but most of us have faced at least one gym leader that fucked our shit up when we faced them, mostly because it was the first time we were battling against them. Which ones gave you the most trouble? In my case

>Misty
Like many did, I picked Charmander on my very first playthrough of Blue and her Starmie Bubblebeamed my team into submission.
>Bugsy
I had a harder time against him than Whitney the first time. I had picked Chikorita, and I let his Scyther use Fury Cutter a couple of times.
>Winona
I had picked Treecko and I didn't have any Fire type pokemon in my team, so her Skarmory took care of a good part of my team, and after one or two Dragon Dances her Altaria didn't have much trouble beating the rest of it
>Maylene
Again, I didn't have any Fire or Fighting type pokemon with me, so her Lucario beat me the first time
>Fantina
I skipped D/P and played Platinum first. At that point in the game, her Mismagius cuaght me off guard
>Lenora
That Retaliate
>Elesa
Well, my ground type attacks will surely... Electric/Flying? Ok in that case I'll go for... The fuck is that Volt Switch thing?!
>Cheren
lol Lillipup? This is going to be... oh it knows Work Up... Why the fuck does that little shit hit so hard?!

No Gym leader in gen 6 gave me any trouble, but I'd say Clemont provides the best battle in XY