Games that I have played:
Prototype sucked IMO, it was just a repetitive hack n slash with some flare.
MMOs games: Want some company while taking on hordes of enemies?
WoW- yea, it is a good game, even if it's over run by 13 year olds w/ superiority complexes.
CoH- A fun MMO with superheroes, quite addictive.
EvE- A nice space MMO, train while you're offline.
Neverwinter Nights series- dungeons and dragons on your pc, lotsa player created content.
FPS/shooter games: BOOM headshot?
CoD4&5- excellent shooting game, gets a little repetitive.
Battlefield 1942, BF2, BF 2142- Great fun can be had dropping scatter bombs on troops.
Fallout3- One of the best RP/shooter games ever made.
Band of Brothers: Often overlooked, but a good solid game, great story lines.
Bioshock- 5star story, EXCELLENT game!
Crysis- A PC resource hog, but a stunning game.
Farcry 1&2: Some great fun to be had here.
RP Games: Want to be something other than your self?
Elder Scrolls 3/Morrowind- It ain't perrty, but it's a very good story and lotta fun.
Elder Scrolls 4/Oblivion- Ok story, lotsa fun, good sandbox game, a little repetitive.
Fallout1&2- Good games as well, if you like story and a true sandbox game, try these.
Strategy Games: Zerglings?
Command&Conquer (all of the series)- If you like RTS games this series is great. Great story.
Company of Heroes-: Great game, often a little repetitive, online will win you over.
World in Conflict- Single player = blah, but online is so fun. Prepare for the triple-nuke.
Sid Meiers Civilization- A true strategy game made by the best.
Sid Meiers Railroad- Yea yea, kindy dorky but it was fun when I played it, good strategy game.
That's about all I can think of, I'm sure I'm missing a ton of other great games.