Basically, the whole main cast was very good.
Ian and Patrick were especially great in X1 and 2, but as Cyclops already said, Ian seemed bored throughout X3, and lacked his charm.
Jackman, Janssen and Paquin all had interesting characters to play, and for me, they ran with it. Berry and Marsden were not so lucky. They're not bad actors at all, they just didn't have much to do. I especially would've loved to see more of Marsden.
Of the new castmembers in X3 Grammar was fantastic, maybe the best performance of the entire movie for me. Page was good too, but for her goes the same as for Foster: It wouldn't have hurt if we had got to see more of them.
Oh, and why does everyone think Kirsten Dunst is so bad in the Spiderman movies? I think she's a good actress, I look forward in seeing her Marie Antoinette.
I also thought Katie Holmes in Batman Begins didn't look as bad as she could have next to Bale, Cain and Freeman.