IL-2: 1946 - the campaigns are nothing to write home about, as is the A.I. Graphically, the sim holds up surprisingly well. The action, I am told, is all on-line. A weird case of a study sim turning into a survey sim. Pretty much mandatory for WW2 sims.
Battle of Britain 2 - What Rowan should have released in 1999, except for that they were going out of business and stuff. Best A.I. in the business and a campaign mode that doubles as a strategy game in itself. Another sim where the graphics hold up OK.
Falcon: Allied Force - Yeah, '98-'99 was the last gasp of sims. Getting long in the tooth graphically, but everything under the hood is fairly incredible. Best dynamic campaign in the business.
These three will keep you going for a very long time.
I’d also play:
Secret Weapons of the Luftwaffe - not a sim, but is what I feel is the best action fly-around-and-shoot-stuff around. Big, lots of planes, AAA production values.
LOMAC - really only comes into its own with the expansion, which puts Starforce on your system…to me this is an instant NO SALE. Otherwise everything else is done right except the stupid campaigns.
Enemy Engaged 2 - remember, kids, if you are going to bring a sim back from the dead, try to make it better than the mods floating around out there for the first game.
Anything from Third Wire - Have vista? Too bad. Otherwise, graphically sparse, boring campaigned dreck. 5,000 polys for the plane, 200 for the landscape. Bad AI. Bad everything.
Avoid sims altogether, otherwise you’re going to spend 100+ on a decent HOTAS and 100+ for TrackIR, for like two games.