Depends what exactly you want - if you want a realistic study sim (that will have a serious learning curve needing hours of study) then LOMAC & F4 Allied Force are the 2 best.
If you want something a bit easier (& arguably more fun) then look at the series of sims from
Third Wire - they're available cheaply if you look.
Also massively modifiable with hundreds if not thousands of user addons & mods.