I have Games! (And A Meme)
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)
11 Angel Roleplaying Game (7.67)
12 Talislanta (7.62)
13 Buffy the Vampire Slayer Roleplaying Game (7.62)
14 Mouse Guard Roleplaying Game (7.6)
15 Dread: The First Book of Pandemonium (7.59)
16 Prime Time Adventures (7.56)
17 Big Eyes, Small Mouth (BESM) (7.55)
18 Don't Rest Your Head (7.55)
19 Starblazer Adventures (7.55)
20 Tribe 8 (7.54)
21 Fading Suns (7.52)
22 All Flesh Must Be Eaten (7.45)
23 Jovian Chronicles (7.45)
24 Adventure! (7.4)
25 Silver Age Sentinels (7.39)
26 Reign (7.37)
27 RuneQuest (7.36)
28 Call of Cthulhu d20 (7.34)
29 HeroQuest (7.33)
30 Dungeons & Dragons Rules Cyclopedia (7.33)
31 Faery's Tale (7.33)
32 Dogs in the Vineyard (7.32)
33 Star Wars: The Roleplaying Game (7.32)
34 Wraith: The Oblivion (7.31)
35 Puppetland: A Storytelling Game with Strings in a Grim World of Make-believe (7.31)
36 Hollow Earth Expedition (7.31)
37 Ars Magica (7.29)
38 Legend of the Five Rings (7.29)
39 Artesia: Adventures in the Known World (7.29)
40 Heavy Gear (7.29)
41 Deadlands (7.28)
42 Changeling: The Lost (7.27)
43 Monsters and Other Childish Things (7.25)
44 InSpectres (7.21)
45 Dark Heresy (7.21)
46 Kult (7.21)
47 The Extraordinary Adventures of Baron Munchausen (7.2)
48 Castle Falkenstein (7.19)
49 Blue Planet Player's Guide (7.19)
50 Blue Planet (7.18)
Actually, I'm not sure on the Blue planet Player's Guide, so I went with my gut.