My fave line up of x-men had to be the classic early ninties incarnation.
I liked this group mainly because I felt that each one was really different from the other both in powers and character.
Honorary mention has to go to Ice Man, Nightcrawler and Collosus. These characters were big favourates of mine too but I was never a fan of really big teams, a more smaller compact team works better imo for more in depth character arcs and stories.
Characters I never really liked as part of the team were
Angel/Archangel - Useless as Angel, got better when he became Archangel but not by much.
Emma Frost - I don't like her as part of the team, seems wrong somehow.
Psylocke - Just never really found her that interesting.
Bishop - I do like Bishop I really do, he almost made my honorary list, but his time traveller backstory irks me for some reason.
Havok - Prefered him during his ninties run as part of X-factor.
Dazzler - Just wack.