This is a tough one for me.
Archangel was always my favorite when I read as I felt he had a story that could really have had a bigger impact than was allowed. (Apocalypse was my favorite bad guy.) However, due to low "screentime" I would probably nominate Gambit. Gambit holds a strong story and is a pretty well rounded charactor. He is probably my favorite in that aspect plus his miniseries was my all time favorite X-men related story.
Honorable mentions:
Iceman
Psylocke
Colossus
Rogue
Havok if we are including al the X-Men spin-offs and stuff.
Bishop
Wolverine would be better if they didn't parade him around so much like the over-rated meal ticket. He is a solid charactor design, however is story is non-coherant and inconsistant. As well as his "bad assness" level is mostly in relation to writer fanbias.