i Put this list up in 20 min
15 Neverwinter Nights (With Expansions)
14 Victoria 2
13 Civilization 4 (With expansion and standalone)
12 Civilization 5
11 Football Manager Series
10 Star Wars: Jedi Knight 2
9 Europe Universals 3
8 Star Wars: Knights of the Old Republic
7 The Elder Scrolls IV: Oblivion
6 Fallout 3 (With DLC)
5 Mass Effect
4 Balders Gate 2: Shadow of Amn (With expansion)
3 Dragon Age: Origin (With Expansion)
2 Mass Effect 2
1 Half Life 2 (Include episode 1 and 2)
I may have forgot some