These aren't necessarily my favorite games but I had a lot of fun the period I was playing them.
1.Vanilla WoW (I consider this the ultimate mmo experience with no equal, call it nostalgia or whatever, nothing will ever come close for me)
2.Age of Mythology: The Titans (although obviously inferior to vanilla AoM in terms of gameplay, I was at a better age to enjoy it and the community was awesome, good old rts-sanctuary helped me through puberty 11111111111111 (hope someone will get the reference)
3.Lineage 2: The Chaotic Throne - Gracia Final (This mmo had a pretty rocky history. I've played it for long but I think around this chronicle it started becoming interesting)
4.Battlefield 3 (I was lucky to have many friends playing this one, too bad it had so many technical problems)
This last one is going to be a little odd, but it's probably my favorite game ever. It's a Lucas Arts shooter from 1997 called Outlaws. I'm a huge western buff and this is spaghetti worship at it's finest.
Special mention goes to Rott (Rise of the Triad). I was too young when this came out but I played some years later. Awesome game.