It's quite hard to answer such topic taking into account old games since I don't remember how much I paid them, and most of the time I was pirating stuff since I was a student.
I guess the question could have been: "Games that gave you the most bang for your time"
Sticking to the bang for your buck ... my top, from lowest to highest:
- Dead Space 1: Only 16 hours, but one of the few games (if not the only one) who was able to scare me. And a game often on sale. A pity the series went downhill after.
- Deus-Ex/Alpha Protocol: Bought for nothing during sales. Very enjoyable games. About 40 hours each.
- Borderlands 1/2, L4D 1/2: Spent countless hours playing those with friends. Topped 140 hours under Steam with BL2 only.
- X-Com: Enemy Unknown: Spent countless hours. First game I dared modding/bug fixing through hex editing to increase its longevity even more. Soon to reach 100 hours
- Skyrim: Well its TES. Although it's seldom on sale, with modding its just a delicacy. 60 hours and I barely scratched the surface because I always put it on hold to take care of my game backlog ...
- Batman Arkham Asylum and City: Great games on many aspects (not counting the fact they are on sale so often for next to nothing). Only 22 hours on Asylum, but still spending countless hours playing City in the Joker/Penguin arena to blow steam off: about to reach the 100 hours.
- Everquest: Obviously ... can't say how much time, but it ate my life for several years.
- Mass Effect 1/2/3 + Mass Effect 3 MP: Even with the butchered ending, this epic story will always hold a place in my mind/heart. And the MP was just icing on the cake. The only game that gives me a reason to keep Origin installed. Difficult to say how long since I played each twice, sometimes with boxed versions ... most likely over 300 hours everything (SP+MP) included.