I'm going by the 90s versions, because that's the era I love.
90s Rogue was AWESOME before they ruined her for the movies. I hate that they got rid of Ms. Marvel's powers. Her entire personality changed, and I don't care for the character in Evolution or Wolverine and the X-Men.
Honorable mentions to Cyclops, Jean & Wolverine. The entire 90s cast was awesome.
One of my least favorite X-Men is actually the Professor. His character always put me to sleep, and I liked it when he faded into the background. Again, I didn't like the prominent role he had in the movies.