This is gonna be hard. I have so many games (like 50 >_>) and I love most of them.
I can't really rate any of these over another, so these are all 1. for me:
Half-Life 2
Max Payne 2
Diablo 2
(All 2s, it's a conspiracy!)
Here are some honourable mentions:
Metal Gear Solid 2
Resident Evil 4
Resident Evil 0
Half-Life 1
Half-Life 2: Episode 1/2
Diablo 1
Now post yours!