Liepards. I thought they'd be nice at first. Nice Spd, decent offenses. 1st blow to that thought: every Liepard spams Fake Out. 2nd and final blow: their tiny puddle of a movepool doesn't allow much, hence common Fake Out.
On a more teeth-grinding scale, Mew. Back in the days, I was the one using Mew and glitched up movesets against friends. Now that I taught said glitches and said friends are using them against ME, that pink floating thing doesn't look so awesome anymore.
On an awesomeness upgrade, ANYTHING with Mimic/Assist in Generation IV games. ... Okay, almost everything. Delcatty will always remain a waste of Pokemon slot. But Infernape with Eruption or Weavile with Fusion Bolt rocks.
Pretty much all of Generation 5. hahaha The games came out and poof! New favorite gen, despite hating it. xD
Anyways. I'd say the biggest change of experience is Garbodor. I used to hate the thing, despite loving Gen 5. I listen to a Pokemon Podcast called It's Super Effective! and the host really liked Garbodor for a time. Seeing someone actually like him made me kind of take a step back and try to evaluate the Pokmeon, give him a chance. And I really like him. Decent battle-wise, better set up-wise. He's like this big, giant, ugly teddy bear. Misunderstood.