Even if it's just another FPS, F.E.A.R. was a lot of fun thanks to the creepy scripted moments and the AI that exploits it's numbers. There are still moments of silly FPS cliches, cheesey horror plot twists, and the usual modern conventions, but it's a solid game with neat features that doesn't demand being run and all the best settings to enjoy.
The Elder Scrolls IV: Oblivion is a great game with way too much to do, but a lot of it's charm comes from it's voracious graphics. It's a huge system hog, but it's single player nature and the possibility of mods makes it the sort of game that may be the last you play for another year or two.
If you'd like to dip into the bargain bin for a short, quirky strategy game, try Darwinia. It's not terribly technical, or hardware intensive, but it's cute and has a great retro charm. It's very short, thought, and that's probably not what you're looking for.
If you don't mind horrible, mind-rending addiction, World of Warcraft is not only a superbly crafted MMO RPG but a desperatley social experience that will make you love not having a social life outside of a "guild" of cow people. It's really a beautiful world to get lost in, but keeping up with other players demands a lot of time, and the deeper content can demand several consecutive hours to complete. I've never been so happy to waste my time, but at the same time I'm much happier not playing and spending all of that time talking with humans and playing a variety different games on different platforms.