What era are you looking for? A lot of these games are a little older now, but there are a few options all with good communities. Also, are you looking for something fun and easy to play, or complicated and hard? Here are a few options anyway.
Generally good community for all your flying needs: www.simhq.com
Modern air combat. Not totally detailed sim: Flaming Cliffs 2 (requires lock on) - www.lockon.co.uk
Made by the same company, but not got fighter aircraft yet - just ground attack, is the DCS series: www.digitalcombatsimulator.com
the community for lock on & Dcs:
http://forums.eagle.ru
There's also Falcon 4.0. Very detailed sim, graphics show their age a bit. Not sure about the community. I picked up Falcon 4.0 Allied Force very cheaply in the UK.
For World War II there's Battle Of Britain II: Wings of victory.
community: www.a2asimulations.com
IL2 Sturmovik 1946 is a great World War II sim. Plenty of variety. Community:
www.mission4today.com
A more game-y version of IL2 is Wings Of Prey. This is available on Steam, and easily available in stores. Don't really know much about the community.
If you like World War I, there's Rise of Flight which is a very good-looking sim.
www.riseofflight.com (I think) Their community is pretty helpful in my experience.
If you like helicopters there's EEAH & EECH, both available on www.gog.com
for a more game-like helicopter game, there's Apache Air Assault, by the guys that did Wings of Prey.
Lots of these games have been heavily modded/patched by the community, but I won't go into detail on that in this post. just be aware that some of these patches, although community made, are pretty much essential.
You should be aware that this type of game in the main is more CPU than GPU dependent.
Sorry for the long post. Hope it's not too "wall of text"y