Over G Fighters is very good if you are looking for a more realistic flight game on the Xbox 360, but is definitely something to skip if you are looking for a arcadish Ace Combat style game.
HAWX I hear is also fairly fun, although I haven't played that to truly have an opinion about it.
The Blazing Angels games are fun arcade-style WWII flight games, and if you want something more realistic in that realm there is always IL2 Sturmovik: Birds of Prey.
Finally, if you want a fun arcadish space-flight game, Project Sylpheed is a good choice.