For me this is a no-brainer. That level where you are relentlessly pursued by those giant nautilus shells (I think they're named Nemo) is way ahead of anything else in the game. Actually, I can't think of any other level in any other game that makes me feel more on edge than that DKL level. It's madness. Those Nemos make Croctopus Chase seem like a splash in a wading pool by comparison.
The reason Nautilus Chase (which is the name of that level in DKL) is harder than Croctopus Chase, is because the Nautiluses actually CHASE you, where as the Croctopuses simply follow a path, but then, calling the level "Croctopus Moving On A Set Path So Get Out Of The Way Or Let It Pass You" would be stupid.