• Gambit (X-Men TV series era) - amazing catchphrases, hot cajun accent. He has pretty cool charming and kinetic powers, but who cares about those?
• Nightcrawler (Excalibur era) - teleportation and shapeshifting abilities
• Azazel (can't remember the exact time period, but during the story arc where he called his children to sacrifice themselves and led the X-Men to believe he was Satan himself.) - unlimited teleportation abilities, and he's immortal. That helps.
• Mystique (yeah, I want the whole family on my team. Big deal.) - master manipulation abilities beyond just shapeshifting.
• Wolverine (Dark X-Men era) - combat/war experience, badass attitude, healing abilities if someone actually manages to touch him.

Honorable Mention:

• Shadowcat (Excalibur era) - her "powers" don't interest me, but her technological prowess does. As well as her possession of Lockheed and Widget.