Falcon 4.0 or the slightly updated Falcon 4:Allied Forces if you want to fly in online (squads can be great fun.) This is still probably my favorite because of the dynamic campaign, this is honestly like a strategy game in it's own. You can even play an entire war here without actually ever getting in a jet. Enough can't be said about this feature, it's an unargued point in the flight sim community.
The downside is you really only get the to fly one jet, the F-16 obviously. There are variants however, and the other area this series really shines is in the avionics so the different variant's (F-16A/B, F-16C/D, Blk 40/42, Blk 50/52, MLU's, etc) are actually quite a big deal (and everyone's got their favorite variant.) There's also additional map packs (dynamic campaign works with them), hires textures for everything, and mods like Red Viper which almost make it an entirely different game. However this really makes this game pretty involving to maintain. Red Viper is probably my favorite game play wise, but "the dance" to install everything and maintaining it when stuff updates just burned me out.
So that brings us to Falcon 4:Allied Forces. This is the newest official release and the version I would recommend going with. Some things got axed from the 4.0 release, but overall it's the much smoother version. Hires packs are available here as well and I highly recommend them - these are easy to maintain as well. The dynamic campaign is here, the avionics are here, and decent network code is here and flying with a squad of people (especially if you fly together regularly) is extremely fun and challenging.
Unfortunately I've been out of the loop really and haven't flown in over year, just don't have the time - but I still get the itch and almost installed Allied Forces over Thanksgiving. I'll have to look into FreeFalcon5 too, looks very nice. I highly recommend investing in a
HOTAS setup if haven't got one already, it's a necessity IMO.
If your not looking for something that sim-like, ummmm... I dunno.