Hmm. Depends which angle I look at it from.
In terms of recent years (this decade), the only games I've completed more than once are:
Need for Speed: Porsche 2000
Diablo 2 (and that wasn't properly completed, i.e. not Nightmare)
Quake3
Motorhead
It's not that I don't play a lot of single player, or see them through to complettion - in fact in the past couple of years I've completed over 40 games. It's just that once I've completed a game, I'm very rarely motivated to go back to it again when I could be trying something new. Games like KOTOR, Deus Ex etc, would be fun to play a game, but with so many great games I haven't even tried yet, it's hard to find the time.
Now, if we look at a longer timeframe, still on the PC, it is probably Quake or Quake 2, both of which I've completed about 5 times. Or maybe FIFA World Cup '98, something like that.
Finally, looking at other formats, it's much harder to guess. Console games tend to be a bit easy/short. I've completed Streets of Rage 2 a couple of dozens times probably. Streetfighter 2 (in various incarnations) maybe a couple hundred times. World Cup '94 on the SNES, again, several dozen times. Going back to the 80s, Target Renegade on the Amstrad CPC about a dozen times.
Anyway, here's some the reasons why I don't complete games more than once anymore:
Morrowind GOTY
Oblivion
Football Manager 2006
Civilization 4
Neverwinter Nights Deluxe
FIFA/NHL/NBA/Madden 2006
Freelancer
Dungeon Siege 1 & 2
Fallout 1 & 2
Baldurs Gate 2
Starcraft & Broodwar
^These are all games I haven't played yet. Why play games I've already completed again, when I've got all these classics on my shopping list?