As I said in chat earlier, I'd vote Sandbox right off if it weren't so damn hard to learn. I figured out the essentials of all the other editors in about an hour or two. It took me about the same length of time digging through tutorials to find out how to place some friggin' terrain in Sandbox.
^This. If Sandbox was a bit easier to handle, not only I would've probably gone for that. Considering it's a quite an annoying process to learn stuff on it (although I must admit I didn't try any tutorials back then), Sandbox loses big parts of it's appeal.
I haven't used it for about 1 and a half years though, maybe I should try again, I'm almost certain I'd have more success on learning it today.
FC2 was the easiest, Forge is annoying, and LBP just seemed pointless (2D).
I agree, Forge is annoying, but I don't consider LBP's editor as pointless, infact it's a pretty effective tool, despite being 2D.