Flashback because it was awesome at its time.
Starcraft because it's addictive
The John DeFoe quadrilogy (5 days a Stranger, Trilby's Notes, 7 days a Skeptic and 6 days a Sacrifice) of Ben "Yahtzee" Croshaw because of its great story.
Ancients Domains of Mystery because of the variety of choices and possible roles you can take. It's the only game I know where one can change from the side of good to evil and then to good again every time one wants to do it (though through some effort).