I don't value my purchases just by game time, it's also about quality of that time. Some games are just very addictive, but once you're done with them you realize that actually they weren't much fun after all. Also, some games offer an experience that sticks with you, even when not playing.
Bearing this in mind, these are my best purchases:
-Morrowind GOTY, ?7, sank lots of hours in that, much fun and a great experience
-Deus Ex, ?3,50 (used copy), again: sank lots of hours into it (played it 5 to 10 times through), lots of fun and a great experience
-Mount & Blade Warband, ?5 during Steam sale, have sunk 246 hours into it already and will probably return to it one day. Also great fun, though not a very interesting experience.
-Red Faction Guerilla, ?3,33 (as part of a 3 for ?10 deal), great fun and played it for many, many hours (not entirely sure how many, but probably about 100 hours or more).