Utility classes are the most represented in raids right now (8 spots for bards/chanters) so that's your best bet to get in anywhere. Shaman (mystic moreso) always seem to be needed at high end guilds. Wardens were recently improved and have had their desirability increased.
If you're shooting for a casual raid guild you could get away with most any other class if you excel at it, but utility is the best bet for average players.
Make sure it's a class you enjoy though!