Wow, this is actually quite hard. I'm probably going to post this then want to change my list, but offhand I'm going to go with (In no real order)...
Civ. Because Civ has sucked up more hours of my life than I care to count. I didn't find Civ III nearly as addictive as the second installment, but 4 is shaping up nicely from the looks of things.
Doom. Because shooting people in the face is always fun.
Mario. While I'm not the biggest fan of Sunshine all of the other games have a pick up and play appeal that's good for 10 minuites or a couple of hours.
Honourable Mentions -
Knights Of The Old Republic. Both good games, problem is that there are only two of them.
Max Payne. As above.
Broken Sword. I thought that the 3D incarnation lacked the character of the 2D versions. The animations weren't varied enough, voice acting was damn good though.
SI's "Manager" series. All have had waaaaay too much time put into them, despite being little more than glorified spreadsheets. Love it.
Tony Hawk's Pro Skater. Loved them up untill 3. Bought 4 and felt it was a bit of a chore, then got bored.
Hitman. We really need a proper 3rd game. The only thing more fun than shooting people in the face is dressing up then shooting people in the face. Or poisioning them. Or Garotting them. Or whatever.
Zelda. Not gotten around to Wind Waker yet so can't really comment.
And probably about a billion others.