It surprised me how different people's opinions are. Let's settle this once and for all with a poll. You can select two worlds.
My top 5
5 - casino
A visual mess. Has nice levels but also a lot of filler.
4 - sw0mp
Confusing and somewhat ugly (though that's the point). Has decent levels.
3 - ice
Great sense of cohesion while having varied locations and objectives. Beautiful music. A bit empty though.
2 - temple
Nice sense of cohesion and adventure. The Skyrim style climbing is fun. The cloud affecting a large part of the world is great and I would've liked to see similar things in other levels.
1 - s p a c e
Best theme and the most varied/creative objectives.