I voted for Storm, Colossus, Kitty, Cyclops and Iceman.
Yes, Storm got lots of lines in X3, but they still mostly just use her as a weather machine.
You see Iceman grow, loterally, but his character doesn't get much of it at all.
Cyclops, well, it's been said enough. They ****ed up.
Kitty was just there to showcase a cool-looking power.
Many other characers should be on this... it would probably be easier to ask "who received development", because the answer would be limited to like 5 -7