Do you want a Combat sim, or a Flight Sim with fighters? I'll go ahead and make a few recommendations:
FSX is really cool, and the expandability is almost unrivaled (add-on aircraft, scenery, missions, etc...some are free, some you gotta buy). It's relatively realistic, and they have an official expansion pack (Acceleration) that includes an F/A-18 and you can do carrier ops (somewhat tricky). As far as a "whole package" sim, this is where it's at. Only downside is that even if you do get the fighters, there's nobody to fight.
X-Plane is another one that has been pretty good at giving you a wide variety of aircraft and scenery. I haven't bought a new version for a couple years, and it always has felt a little too fake. But, judging by this video
, it has come a long way and I'm excited to try it out again.
If you're looking for a combat sim, look no further than LoMAC. It's a couple years old, you can get it cheap, but it is awesome. I can't get enough of buzzing around in the A-10 mowing down tanks with that monster cannon.
There is also an open-source sim called Flight Gear that looks promising, but for some reason I have never been able to run it on any of my systems. If you've got a thoroughbred pc, you might give it a shot. Free stuff is always good, right?
Hope that helps you some.