Star Control 2. Game lore is massive and deep, and is given out in small chunks, never overwhelming the player. Mass Effect actually borrows from this game a lot.

Tie Fighter. If I had any doubts fighting for the Empire, they were all gone in just five of six missions. After that, I wanted to blast as many traitorous rebel scum from the space as I could!

Betrayal at Krondor. One of the best RPGs I ever played. Writers could challenge R. Feist himself!

Full Throttle. The finest LucasArts quest ever, with great characters and story.