Depends what you've already played on consoles:
Exclusive:
Hard Reset
Mount & Blade
FTL: Faster than Light
Total War
Amnesia: The Dark Descent
Darwinia
EYE: Divine Cybermancy
Civ V (get it with the expansions when you get it; they really add to the game)
Multiplats worth purchasing and modding beyond recognition:
Skyrim
Fallout 3
Oblivion
Crysis
GTAIV