Ok, here are my recommendations. Some are old games, some are recent, but all of them are good (IMHO):
Civilian Flight Simulators:
- FSX
- X-Plane 9.0
Combat - Prop planes (WW2):
- IL-2 1946
- Battle of Britain II: Wings of Victory (fantastic... AI is brilliant, great campaign)
Combat - Jet planes
- Mig Alley (feels like props, but with jets; dogfights are fantastic. One of my favorites)
- Falcon 4.0 Allied Force (campaign is still unrivalled in immersion)
- Lock On: Modern Air Combat (great physics, specially if you grab the expansion)
- Strike Fighters series (pretty light in the sim department, but with lots of nice mods)
Combat - Helicopters
- Enemy Engaged: Comanche vs Hokum (fun, fun, fun. If you get it, get the community patches; if you can, get Apache vs. Havoc too)
Combat - Space
- Independence War series (also known as I-War... great space sim)
- Evochron Renegades (indie, but very, very good... even supports TrackIR)
- X3: Reunion (takes a lot of time commitment, and you'll likely play for a while before seeing some action, but it's pretty good)
Also, feel free to visit SimHQ for other options. Look for me in the forums, if you want (I'm "Wolfox" there).