I believe that I read practically all retro-clones out there by now (but I’m still on watch for new ones, like Crypts & Things and Dungeon Crawl Classics, although the last isn’t actually a clone).
Swords & Wizardry is still by far my favorite and the one responsible for creating in me an interest for the first editions of D&D (together with Jame’s amazing Grognardia). S&W has that indefinable quality that instantly ignites my imagination (call it whatever you like: nostalgia, “magic” etc) and I’m still surprised of finding myself reading again and again my White Box and Core Rules version.
So, I couldn’t let pass the opportunity to mention the new 4th Printing of S&W that just came out from Mythmere Games (the PDF is free). It’s probably the Printing with more alterations so far, influenced by both the White Box and the Complete Rulebook (this last one from Frog God Games) – it includes even rules for using Holmes’ initiative system. I must admit that the new Ability Score rules (taken, I believe, from Supplement I) are more detailed than I desire and is obviously that, although still a very rules-light game, S&W has grown in complexity.
Another S&W game that I believe is worth of attention is Redwald. A project in progress by Lee Reynoldson (and is in need of playtesting!), Redwald is a very flavorful and interesting S&W setting based on the old anglo-saxon world and its myths. It has new races, classes, excellent monsters and very intriguing subsystems, showing how far the basic rules of S&W can be adapted to suit a Dungeon Master’s need. Even if you don’t play S&W, the ideas behind Redwald are great to any fantasy campaign.