Single player: In no particular order:
1: Farcry series
2: Bioshock series
3: Doom 3 onwards
4: Fallout 3 series
5: Half Life series
6: Portal series
7: Crisis series
8: Cryostasis
9: Trine
10: Timeshift
Special mention to Stalker, Rage, Metro 2033, Fear, Alice, Madness returns, Borderlands.
If I had to pick only 5, it would be Farcry, Bioshock, Fallout 3, Half Life 2, Crisis.
Multiplayer: Farcry, Call of Duty.