Since I can't compare between genres, I will make a list like everyone else:
RPG: The Witcher 2
MOBA: League of Legends
RTS: The Lord of the Rings Battle for Middle Earth 2
MMORPG: Most time played: Wow. But quit some time ago. Looking for a new one to fill the gap though :P
Action: Metal Gear Solid 2: Sons of Liberty or Metal Gear Solid. They are equal to me :P
Honorable mention: Assassin's Creed 2