Are you looking specifically for a WWII flight sim? IL2: Forgotten Battles is a good one, and even better if you get Pacific Fighters to go along with it. It can either be merged with IL2:FB as an add-on or installed and played separately. Apparently Aces High is pretty good, but I've never played it so I couldn't tell you myself.
Some modern flight sims that are pretty good are Lock-On Modern Air Combat and Falcon 4.0 Allied Force.
I'll just move this to the IL-2 Pilot's Lounge, by the way.