Title pretty much says it all ; )

Cyclops...and its not even close. But that has a lot to do with how much focus he was given.

Most characters have moments of development, but its either forgotten about as soon as the writer leaves that book, or when the character is moved to a new group.

Scott though had a very consistent path of development from Whedon where he really took charge of the team and started breaking more and more with Xavier, throughout Decimation where he became more and more of a hardliner due to constantly worsening circumstances, then onto the revolution where he finally became more of an outlaw. Its slow burn development done right, and unlike many other "developments" for other characters, his stuck with him.

Wolverine and Storm had the most development during the Claremont era. They were the only two characters that Claremont wrote for the entirety of his run. Storm seemed to receive the most development out of the two, with her going from a woman who fancied herself a goddess to the tough as nails leader of the X-Men. Magneto also received a lot of development in the 80s where he went from bland villian to Holocaust survivor. Cyclops and Rogue had the most character development in the 2000s.

80s Storm, Wolverine, Magik, Magneto, Rogue, Mystique, Shadowcat etc.

Currently, Rogue. She leads Uncanny Avengers and written well too. And she just got married to Gambit and they both will have their own ongoing series.