Kotor 1 and 2, combined I think I've played them around 15 or so times.
Dragon Age: Origins. Not entirely sure how many times I completed it, maybe 4 or 5 times. Steam says I've played it 156 hours, so that seems about right.
Loderunner 2. I've lost count of how many times I've completed it and how many hours I've played it. The replayability value of it is just immense. Completed the levels in different manners and trying to beat the already set times or your own times are fun. Or imposing additional restrictions to make for an even greater challenge, like doing a marathon run(no saving), while still trying to do it as fast as possible. Good times.
Also, I'll have to mention World of Warcraft. I've logged plenty of days, not entirely sure how many, but I'm pretty sure it account to more than a couple of months.
Also, I feel Baldur's Gate 2 and Planescape: Torment being added to this list in the coming years. I've only recetly discovered both of them and currently on my second playthrough on both of them as well.