[Dr Kain]

Personally, I thought most of the casting for the X-Men movies was terrible.

Patrick Stewart is too nice of a dude to be playing Professor X. They made him a grandfatherly character when Professor X is an asshole trying to force his views on others, but he respects others existing. McAvoy portrayed him much more accurately.

Marsden had no range of emotion at all as Cyclops and even less personality.

Berry was a completely miscast as Storm. It's like they only picked her because she was the only big name black woman they could get instead of someone who could act. Not to mention sometimes she had an accent, sometimes she didn't, and they completely underutilized her.

Paquin's Rogue never once called people "Sugah," she was severly underpowered, couldn't even fly, and whined all of the time. Rogue might lament her abilities even know and then, but she doesn't cry about them every few minutes.

McKellen was too damn old to be Magneto.

Stamos had no reason being Mystique either. I've seen trees be less wooden.

Jackman and Jensen are the only ones I would say were spot on casting as far as personalities go, though, Jensen got worse as the series went on.