I rate Crack the Skye as their best by quite a margin, followed by Blood Mountain.
Ofcouse, these are the only two Mastodon albums I own...so that kind of makes sense.
Based on what is on Blood Mountain, and the discussion of others in this thread, I think CTS will be a one-off. It appeals to such a narrow base of fans (again, from what I've read), I can't see them going down that road again. Shame really.