10.) Adventurer, Conqueror, King: Taking the greatest gaming system of all time (B/X) and reconfiguring it to make sure the math makes sense all the way up to the domain level?
9.) Basic Fantasy: It isn't just the B/X inspired simple gameplay, or the modified sections such as race and class, it's the ridiculously supported online download section.
Ambition & Avarice: This is the third wave OSR, the new wave. Taking modern design and layout sensibilities to a new product based on an old idea, comes a game that is really a fresh look.
8.) Castles & Crusades: D20 D&D simplified? It's greatest strength is that any d20 material requires no conversion. Fast, and the SEIGE system is pretty ingenious.
7.) Labyrinth Lord: The Gold standard in free retro-clones. One minor advantage over B/X is that all the text is in one book. Of course all the XP tables are redone. Labyrinth Lord compatibility is code for "This is compatible with D&D!"
7:) Lamentations of the Flame Princess: A beautiful treatise on the nature of Dungeons & Dragons. A smooth, intuitive ruleset. No hiccups, no bumps. Great for lower power, real world, weird campaigns. Metal artwork. The edition everyone loves to hate.
Dungeon Crawl Classics: The edition of the game Goodman wants to play! Weird dice and weirder random tables makes, much like the game it ties with, for a game focused on "A good time at the table!" rather than rules that conceptually make sense or focus on verisimilitude.
6.) Advanced Dungeons & Dragons: "Random chance plays a huge part in everybody's life." -Gary Gygax
5.) Original Dungeons & Dragons: Well, if this doesn't inspire you to do your own imagining. . .
4.) Pathfinder: This is the present of gaming. Pathfinder players are possibly the largest group of publically playing players of any edition, ever. How's that OGL working out for you now, WotC?
3.) BECMI: The version they won't put into print, because no one will ever need another version of D&D again!
2.) Hackmaster 4e: The first original retroclone does Dungeons & Dragons better than nearly any version ever released. Really! It is the greatest!
1.) Basic/Expert: There is no version that is both a better introduction to role-playing, an effective tool for use while playing, and something that supports your imagination.