If you like variety and jets from the 1960s and 1970s, then there is a great series with tons of free addon aircraft available: SFP1/WOV/WOE
These sims are a lot like Jane's Fighters Anthology with an upgrade to 3d graphics and realism with a modest downgrade to AI and online MP.
Go here for info and forums:
http://www.thirdwire.com/Strike Fighters Project 1 (available from Circuit City for $5, you can't go wrong buying and trying this one, but the stock map is a fictional desert terrain)
Wings Over Vietnam (new distributor, but same game engine with historical Vietnam terrain and more stock aircraft, about $20 at Circuit City)
Wings Over Europe (follow-on to WOV... but with Germany, the F-15, the A-10, MiG-23, and MiG-27 added... kind of a LOMAC lite, WOV can be merged automatically merged in during install, this release is brand new and goes for about $30 in retail stores)
All of the user mods for these sims are interchangeable. It is best if you own all 3 so that you have access to all of the stock files which may be necessary for some mods. There are too many mods to list, but include everything from endless aircraft variants to terrains and campaigns.
The stock aircraft list is good when you combine all 3 sims (some aircraft are non-flyable out of the box, but cockpit mods are available that make any and all aircraft flyable):
A-4B, A-4C, A-4E, A-4F
A-6A
A-7A, A-7B
A-10A
An-12
AV-8A
B-52D
B-57B
C-130A
F-4B, F-4C, F-4D, F-4E, F-4F, F-4J, F-4M (Phantom FGR.2)
F-8E
F-15A
F-100D
F-104G
F-105D
Harrier GR.1, Harrier GR.3
Hunter FGA.9
Il-28
MiG-17, MiG-17F
MiG-19S
MiG-21F, MiG-21MF, MiG-21PF, MiG-21PFM, MiG-21PFV
MiG-23M
MiG-27
O-1E
Su-7BM
Tu-16
Tu-22
User made addon aircraft generally equal or exceed stock aircraft quality and include almost the entire Mirage III and F1 families, the F-14, F-16, and F/A-18, almost every USN and USAF jet fighter made from the end of the WW2 to the 1970s, and plenty of MiGs and Sus as well. There is a large number of WW2 aircraft and terrains available, but the game does not fully support prop aircraft yet and the overall quality of most of the WW2 addons is lower than the jets (a lot of them have 3d models converted from other games that had poly count restrictions).
The game is still under development by the original programmer. Every time a new version is released, the older versions are patched to be compatible. For instance, WOV added support for carrier operations and WOE added support for HUD and CCIP avionics and 3d clouds. People who only have SFP1 now have all of those capabilities and only need to buy the later releases if they want access to the new stock planes, ground objects, terrains, and campaigns. There is a WWI version on the verge of release, but so far no distributor has been found for it. There are already some biplane addons from the pre-WW2 era.
While this sim series is great out of the box once patched to the current revision and has a ton of great addons, it does have a lot of limitations:
1. AI is nowhere near as bright as it could be. It is fairly strong in the dogfighting part (some of the best AI dogfights I have ever had in any sim), but fighters assigned to a strike mission or flying home will completely ignore attacks making them cannon fodder. Wingmen under your command never respond quite the way you want them to.
2. Online MP is very fun and I have been hosting a 24/7 server at Hyperlobby for a couple of years now, but is very limited compared to other sims. 2 modes: dogfight and co-op. Dogfight is great mindless furballing fun, but there is no way to rearm or refuel, so you have to
out and get a fresh plane every now and then if you don't get shot down first. Co-op is cool, but you can only select the mission type (no way to make custom missions the way you can in single player) and the world is a lot more sterile (only enemies around the targets, no clouds) to try to keep down lag problems. There is no way to create a dedicated host for co-op.
I am fanatical about this game because I am an F-4 Phantom Phanatic and love MiG-21s as well. There is simply no better sim out there for flying these two aircraft in combat. The stock FMs are a little dumbed down, but I proved the game engine was capable of matching published data by making an F-4B FM that almost nails the charts in the pilot's manual. I still think Aces High is the best sim for WW2 air combat, but for jets I spend most of my time on SF/WOV/WOE. The best part is that the game keeps getting better by the day as modders figure out more ways to use the features available via the ini files and the developer (TK of Thirdwire) continues to add features via service packs and patches. If TK ever gets around to supporting online MP properly, I probably wouldn't play any other flight sim.