favourites: badlands and ash. i just like their style. badlands is awesome for decoration, and ash...well dunno, i just like it. but you really have to be aware of the bad aspects of the tileset (doodad situation, terraindeco only unbuildable, but en mass, limited heigths) in order to choose the concept wisely.
i dont like desert and space. desert sucks for decoration due to many "dead terrans" things but only few natural doodads. plus, i dont like the colours somehow. space? havent made too much in it. i think it has very limited decoration possibilities.
desert, twilight and space are the top 3, but actually all ensures a different style of constructing and designing the map
but i don't like ice very much, because the walling problem that epidion mentioned so in icemaps the first way to make a map is to build on the dirt-water-structure-grass, the second is the snow-high-snow-ice