In order of progression:
1 - Gravity Garden Zone
Also the best concept in my opinion, coincidentally.
2 - Cloudy Domain Zone + Heavenly Tower Zone
A good combination, but not too many rainbows if they are going be there.
3 - Cryonic City + Frost Factory
Now this is a better combination that I would very much like to see, preferably if this combination will gravitate more towards the 'city' part with some of Frost Factory's ideas.
4 - Honeyhive Zone
A theme unseen in SRB2 ever before? Yes please!
5 - Blue Lagoon + Zany Ziggurat
I wanted to vote for just Blue Lagoon, if only because it served as a break from all the factory/building/temple-containing ideas, but apparently we can't have that anymore.
6 - Haunted House
This has some really amazing ideas and I like the 'no badniks' part.
7 - Photonic Capitol Zone (+ Robotropolis, possibly)
Best final zone idea, if it doesn't use DCZ textures and possibly incorporates some of Robotropolis' ideas. If this could eventually focus more on the buildings (with all the light-based ideas of Photonic Capitol), I'd like it even more!