I gotta say Warcraft 3. Warcraft strategy gameplay is easy to enter but hard to master, the art style is a mix of cartoony and darkness. It has RPG elements with your hero commander units. Its custom game arcade was legendary.
After that I got to say left 4 dead 2, its got coop, its got versus, its got mods, its got characters with own lines. Just like Warcraft 3 it is slightly cartoony with plenty of darkness. It is by far the best shooter ever created.
DayZ - This changed multiplayer FPS for me forever. Played the mod and loved it, played the standalone and community servers. Something hilarious about interacting with other survivors and forcing them to eat a damn rotten apple.
Singleplayer: Arkham City or Dishonored 1