Just in my opinion the greatest member of the X-Men is Cyclops. The story does not always center around Cyclops and he may seem bland but it has been shown that there is more to Mr. Summers than meets the eye.
I believe the true nature of Cyclops and how deep of a character he is was not revealed until the Phoenix Saga when the last 2 members remaining were Cyclops and Jean Grey. Once Jean dies Cyclops shows an emotional response never seen by him in previous stories and subsequently quits the group.
My favorite X-Man would be the group itself. I think as a group the X-Men are great, they have some of the most interesting stories and are one of the best parts of the Marvel Universe.