There are some good flight sims out there, but most are just straight forward "Here is your mission: ...Go blow stuff up" and don't really have much depth to them. However, Ace Combat has tried to actually add an engaging story to each of their games, along with some pretty good cinematics. I think, hands down it's the best combat flight sim available for the major (game console) arena.
The plots seem a little cheesy at times and the in-game hero worship is a bit over the top. Although, a little hero worship can be just what the doctor ordered when you're suiting up for a dangerous mission.
I voted for "Ace Combat 5: The Unsung War" as my favorite. Mainly because I think the story was (overall) one of the coolest in the series. Also, the depth of play was stepped up a notch with some added in-game options. Although, like DB said...it's hard to decide on a favorite.