Now that there are print-on-demand Rules Cyclopediæ, the only purpose retro-clones serve is their original one — recognizable labels that indicate compatibility with a specific edition of The World's First and Most Popular Fantasy Role-Playing Game, Wink-Wink, Nudge-Nudge, 'Ello-'Ello, Hint-Hint.
But if I had to pick one (and it had to be one of the faithful-ish clones, and BFRPG and Iron Falcon and Dark Dungeons aren't even on the list), I've been pretty happy with Labyrinth Lord. It still has some advantages over OSE, like some truly kickass black-and-white art, a decade and a half of published content, and the superior Mentzerean rules for magic-user spell acquisition.