Might as well answer the actual thread question as well for a change. Some of my answers will probably stray a bit from the X-Men proper into the extended X-families.

1. Multiple Man
2. In the interests of shortening this list, pretty much the entire current roster in X-Factor.
3. Wolverine (if only because him existing lead to Wolverine and the X-Men).
4. Beast
5. Rogue
6. Broo
7. Kitty Pryde
8. Magneto
9. Doop
10. Cable, despite being pretty much the living embodiment of everything wrong with 90's comics is still really cool when done right.

I won't keep going though since that list already covers probably 20+ characters. Never really got the Gambit love myself. He can be interesting when done well, but I've been read the majority of X-titles published in the last 10-15 years or so and I can't think of many times where he was used really well in my opinion.