1. DCC, because...well, if you haven't heard of DCC, it's got the best mechanics for S&S I've seen in quite a while (Mighty Deeds and Patrons FTW, baby!)
2. Crimson Blades, because it has the second best mechanics for S&S (I mean the Chainmail-inspired version, but whatever...summoning FTW!)
3. Low Fantasy RPG and The Nightmares Underneath, because both are great for a suitably horror-ish world, with mechanics that reflect this.
4. OD&D, because it started it all and can be great with a suitably open-minded GM (like TheWyzard was on this forum).
5. Mastercraft (the system behind Spycraft 2.0+ and Fantasy Craft), because sometimes you want to have all those modifiers on your character sheet...and because it was doing background packages before they were in vogue, and because it gave easy ways to keep the NPC stats simple even on 20th level - a major failing of 3e, if you ask me.
6. Beyond the Wall, because if you want to run Young Adult fantasy? That's the way! And if you don't want YA fantasy, it's still worth looking at the chargen, the magic system and the GMing chapter...also at the way the classes actually fit the archetypes.
7. Crypts and Things, because you can never have too much S&S rules, and it does nice things with the magic system and chargen.
8. Scarlet Heroes, because when you want a single-PC game, that's the way to go...and because it's got a kick-ass setting (Red Tide setting, which includes the best system for managing your domain across the OSR games I've seen).
9. ACKS, because the class-building and the abilities are great, and it puts more emphasis on the "endgame" (but it's behind Scarlet Heroes, because I like Red Tide's approach to domain management).
10. Five Ancient Kingdoms, because it takes inspiration from fairy tales like 1001 nights, and makes it fun to play.
11. Spellcraft and Swordplay, because its Chainmail-inspired mechanics are a thing of beauty for a world like 16th century Europe.
12. 13th Age, because the One Unique Thing, the Escalation die and the Icons are great ideas...and also because the dagger dealing more damage in the hands of a rogue than in the hands of an warrior is an approach worth keeping in mind.
13. Lamentations of the Flame Princess, because nothing screams "Gonzo" quite like it, because it's got great adventures and because it started my interest in OSR editions.
14. Empire of the Petal Throne, which was the first setting...and what a setting!
15. Flying Swordsmen, because at least trying to make d20 wuxia should count for something, regardless of what anyone thinks about it being successful, or not.
16. AD&D 2e because frankly, it starts to lose the major advantages of the TSR/OSR games without much to replace them (or not until late in the edition, AFAIK).
17. Iron Heroes, which I've never played, but has a good approach to the classes, and suggests ways to resolve some of the issues of the edition it's based on.
18. 3.5/PF, because it's so broken it hurts...and it's trying to be a balanced game.
19. 5th edition, because it's current and it's somewhat better for my style than the previous one, but in trying to be all things at once it doesn't quite succeed at most of them.
20. D&D 4th edition, because it's the best edition that was made for a style of playing that's totally unlike the one I like.
At least it's a 20-items list...though anything over 15 isn't a good number.