My top pick would be James Newton Howard, but just to throw another, lesser known name into the mix: if they still want something "hummable" and Williams-esque without getting Williams himself, then they could go with Ed Shearmur. His Sky Captain score was obviously an intentional Williams Superman homage (to the point of rip-off, really), but it was still a beautiful score. And since I only saw the movie once when it first came out but still find myself randomly humming the theme every now and then, I'd say it was fairly memorable:
I could get behind Giacchino, too, even if I was a little disappointed with his Star Trek score.