Here a few sims that I would recommend:
Flight Simulator 2004 - Best of it's kind. Civilian/Commercial/General Aviation flight simulator. (i.e. 97% of the aircraft flying in the sky at any given moment) After purchase, it's highly recommended to spend some time at
http://www.avsim.com , check out their library, and download some of their "hottest files" listed under this year, month, and day. 99.9% backward compatible with FS2002 add-ons. Download of FSUIPC from that site is highly recommended, as it will allow your add-ons to interface properly with the sim. For more information, PM me. This sim is my area of expertise.
Orbiter - Space simulator: Space Shuttle, and Gemini, Apollo, Mercury capsules, and many, many more. A solid understanding of orbital dynamics, physics, and newtonian forces is HIGHLY recommended. Try out the highly-challenging trans-Lunar insertion orbit maneuvers! Or a full flight from Earth to Mars, with your choice of orbital insertion methods! Want to recreate 2001: A Space Oddysey? Go right ahead!
Falcon 4.0 - Best complex military fighter simulator EVER! start here:
http://www.frugalsworld.com , click on forums, then Falcon 4. Ignore the entire "G2i vs. The F4 Community" mess, and read the sticky posts. Or, if you're lazy, just wait for the soon-to-be-released Falcon 4: OIR. It's all up-to-date. Warning: This sim is COMPLEX!
Lock-On: Modern Air Combat - release date Winter 2003. Download the demo at fileplanet.com or fileshack.com . It's somewhat simpler than Falcon 4, and has the Mig-29 (3 variants). Su-25, Su-33, A-10, F-15.
Enemy Engaged: Commanche vs. Hokum - best combat helicopter sim out there. Highly entertaining and immersive. Highly recommended.