John Fiala's Website

Here's the list of the 50 best core rules, as scored on RPG.net, bold the ones you own. (Picked up from chadu's livejournal.)

1 Nobilis (8.31)
2 Spirit of the Century (8.25)
3 King Arthur Pendragon (8.14)
4 Unknown Armies (8.13)
5 Call of Cthulhu (7.95)
6 Feng Shui: Action Movie Roleplaying (7.93)
7 Mutants & Masterminds (7.75)
8 Over the Edge (7.7)
9 Warhammer Fantasy Roleplay (7.68)
10 Paranoia (7.68)