for me it would go(in no particular order)
1.Fallout 3
this game will be the end of me, and I'm sure of it, I have multiple characters, have explored about 2/3 of the games map, and even if New Vegas has a few things that improve on it(the iron sights, companion wheel, the companion base, more quest, etc.) I still come back to fall out 3 more often because it really captures that feel that your living in a post-apocalyptic hell-hole
2. time splitters 2&future perfect
I couldn't pick between just one of these, I mean come on, both offer pretty freaking awesome multiplayer, a level editor to make your own(you could create a basic death match level in one sitting)
3.mercenaries 1
this game was hundreds of times better than it's crap sequel, the games bread and butter was either completing missions for random factions(most of which were at war with one another, so you had to balance out their moods or risk losing good weapons/vehicles/air strikes) or hunting down "the deck of 52"(see, in this game, north korea threatened nuclear war, so the UN intervened and created a list of 52 of the NK's top generals, so you had to work with the various factions in the war torn country to gain intel on the bounties/support to call in the field) all in all, it was an interesting concept ruined by it's sequel, shame
