this is a hard question in that each had some great moments, and most had some....not so great moments. i think the best high for me was likely gaiman's sandman and related materials (but he's not a typical comicbook author, and i have read all his novels as well and loved most of them) but it's tough to look at the jla stuff morrison did and NOT say that was equally as good, just different. one was much more traditional. i think if i were to come up with an idea and had to have one author pen it for me, it would be morrison. but then, it would depend on the idea i guess. hrm. tough call.
i do agree with part of what pr said--lee was great, but i also think that by the standards of today, his early stories don't match up. but that's not his fault. he started something that others--over years and years--have had a chance to hone. comparing across eras is always tricky, but if you're mentioning lee, you really should be lumping kirby in there as well imo.
oh, an honourable mention to denny o'neill whose work with neal adams in batman pretty much redefined the character and i think sort of paved the way for miller.