I only do the main leves that everyone does (i,e., Dunesfolk, Cracked, Ranine etc) so my experience is limited and there's probably some obscure one that is way worse than any I can think of.

But, from my limited experience anyway, I would have to say that Ranine Reveries is the worst. Mainly because the mobs annoy me (stupidly oversized puks) and there's too much running.

But all in all, they're all pretty crap.