Sonic 3 w/ Sonic and Knuckles (had to get the super and hyper forms of each character, after all) (and even then, several more times)
GTA III (only truly 100% completed it once, but otherwise beaten to the credits... numerous times)
GTA VC (see above)
GTA San Andreas (never got 100%, but 90% is close enough)
Morrowind (well, the damn thing's so big, it's easy to find new stuff to do, then I got the expansion packs)
Oblivion (again, see above)
Where in the USA is Carmen Sandiego (ON CD!) (Sorry, they make such a big deal out of that, I had to put it in)
Having a small library of games due to particular tastes makes this a small, boring list. Having not played through most of them a second time makes it smaller. Having never played all the way through in the first place shrinks it further. And having never even attempted single player on whatever's left (Worms and Worms 2, for example) makes it the list that is is.
And I seem to be the one of the few who mention ye olde games like Sonic 3. I wonder why...
And I know starting sentences with "and" is incorrect grammar. Tough. That's the way I talk, and that's the way I type.