Others please post your own top 20 I am not here to argue you just want to see people's opinions (and maybe get some new ideas of worthwhile games to play) These are all games I have beat or put enough time into to get a very good idea of the game (50-100+ hours)
20. Gothic 4 - Arcania
19. Grandia 2
18. KoToR 2
17. A Farewell to Dragons
16. Titan Quest
15. Dungeon Siege 2
14. The Witcher 2
13. Baldur's Gate 2
12. Wild Arms 2
11. Fallout 3
10. Shin Megami Tensei: DDS
9. Xenogears
8. Everquest
7. Mass Effect (trilogy)
6. Planescape Torment
5. Tales of Phantasia
4. FF7 (yeah top 5 suck it)
3. Suikoden V
2. Neverwinter Nights 1
1. System Shock 2