Depends on the level of realism you want. At the top end of the scale you have titles such as
LOMAC (Lock-On: Modern Air Combat + Flaming Cliffs) which is probably one of the most authentic modern combat sims out there. For the WW2-era you definitely want to take a look at the IL2 series. I haven't played the later titles (Cliffs of Dover), but
IL2: 1946 is a collection of every IL2 title up until recently and is certainly worth a look - the multiplayer community is probably one of the largest you'll find for a combat sim. Damage + flight models are pretty decent and there's an enormous selection of aircraft to fly (many factions: UK, Germany, US, Russia, Italy etc. etc.) and many many missions and campaigns, along with an excellent mission editor.
On the arcade side of things you definitely need to check out
Crimson Skies at some point. It's ancient (2000), no active community, but it's easily a classic. Another favorite of mine is
Echelon: Wind Warriors, more of a cult classic, but there's a small community who still play it regularly. Sci-fi combat sim, physics take a while to get used to, but you can pick it up off Amazon for practically nothing (if you can't find a copy, send me a mail - it's pretty hard to get hold of otherwise!).
