If I were you, I would do the aquatica one first, just because it sounds less complicated. Think of the detail levels required for an underwater level, as opposed to crystal caves or clockwork. As you finish a tileset, you learn new techniques and get new ideas to put into your future sets. Therefore, since I think the clockwork and caves are more "advanced" tilesets, you should save them until last, so that you'll do the best possible job on them.

Of course, you could always work on more than one at the same time, too!