Probably the starters from Gen IV. I mean, I like Piplup, but that's it. I felt like I had no choice but to take it when I was given the choice back in my Diamond. And unlike the Gen V starters, I've never really warmed up to them as much as I'd like to. Chimchar's okay now, but still, I really don't like the Gen IV starters as much as I like the other generations' starter lineup cumulatively.
I like most of the starters, though. Some minor flubs in Gen I (wish I liked Bulbasaur more), Gen II (I'm looking at you, Cyndaquil) and Gen V (no one in particular among the available three, but I felt disappointed when their final evos were revealed) in my opinion, but yeah.