Trying to remember what games Ive played now haha.
1. Call of Duty 4 - Still play it more than any other game, simply the best game Ive ever played online I think!
2. GRID - Used to only play racing games on consoles, this is the one that got me going on the PC
3. Half Life - The original, just remember playing it and thinking "Wow!". Installed it again a few months ago and its definately showing its age now

At the time though...
4. GTA 3 - Played for ages, introduced me to the modding community. Think by the end most cars, guns, clothing, signs etc were changed on my install lol.
5. Asherons Call 2 - Old MMORPG I used to play when I was at school, was a serious threat to my GCSE's. Was never a massive game but had by far the best community Ive ever experienced and any MMO Ive played since has failed to get me as involved
Theres probably others I'll remember later lol, but those are ones that stick in my mind. COD4 is a definate fave, the other 4 are in no real order, they all have different bits I enjoyed and couldnt choose between them!