I chose the original boxed set--I liked a lot of elements about the revision, but ultimately I thought it lost more than it gained. That said, I've flipped through the Dungeon portion of the Dungeon/Dragon crossover, and I like a lot of what I see. It corrects some of the problems I had with the second boxed set by making the sorcerer-kings a stronger part of the setting again, and I like most of what they've done. I don't like the inclusion of even more Dark Sun PC races, because they don't feel very "Dark Sun" to me, but then, I wasn't a fan of the new races introduced after the first boxed set, either.
There's no way the Dungeon/Dragon one can be my favorite, because it's simply not big enough to be a complete world. (There's a lot of material I was hoping to read about that they simply couldn't fit into the Dungeon section; I expect the Dragon one to be the same.) But I think it's a definite step up from the second boxed set, and if they'd actually made it a full-sized setting, it's possible (albeit not likely) that it could have given the first set a run for it's money.