Pondering further about that question, I think, I'd go for this order:
1.Tropical (monsoons, lush green grass and nice, brown soil)
2.Taiga (I like the bright, limegreen grass and the biome invites me to build in a more rustical style)
3.Tundra (definitely a challenge, but I love the snow and reddish, withered grass)
4.Temperate (compromise between all choices above)
5.Desert (biggest sandbox in the word x))
6. Grassland (I have only one grassland zoo and I built it just to have all biomes complete. I don't like it after all. The terrain brushes are all too yellowish, no matter if it's grass, sand, soil or even rocks)