Kirk's F-16 isn't too hard.
The KBT F/A-18E can leave some bruises though ;D
Another one would be the Mirage 2000N located on
http://simviation.com/fs2004military79.htm that page. Second one from the bottom.
If you don't mind Commercial Airliners, the iFly 747-400 is BAD on the frames for my PC (3.2ghz, 512 ram).
Also, there's the "Concordeski", Tu-144 Supersonic airliner. Quite harsh on my other PC. Beautiful plane though, and quite fast ;D
4GB DDR2 PC5300; 3.2GHz Pentium D 940, Nvidia 9800GT 1024MB DDR3, Windows XP Pro SP3