Oooh, tough one.... I've probably logged lots of hours with the Quake series, and the Half-Life series...

I'd have to say games that I've played the most would be Monkey Island 1 & 2 and Curse of Monkey Island.

OH and the Old Sierra adventure game series called "Quest for Glory" (1 to 5).

Actually I bought the game when it first came out for my Amiga, and the first one was called "Hero's Quest" until damn Milton Bradley decided to make an electronic version of their poxy board game "HeroQuest", the Sierra game had to be renamed because of it.

Bloody Milton Bradley...