Favorites: Avalon: The music, story, scenery, and midieval theme. A beautiful world. Azteca: the music and scenery especially. Sad ending is impacting. Beautiful world as well. Grizzleheim/Wintertusk: beautiful, the music, environment. Good story and theme. Marleybone: Music, nostalgia, theme, environment, story, and the Jack the Ripper reference
Least Favorites: Zafaria: Boring most of the time somehow Dragonspyre: dull and too many towers. The only part I really enjoy is Malistaire.
Although, I can't say I actually hate any of the worlds :)