I enjoy Toscana and Eifel (circuit) the most, by far. I like Toscana at night, with the fireworks, but DP still hasn't fixed the headlights such that you can actually see up and down hills, so night races are pretty much worthless at this point. If DP would fix the headlights, I'd make a lot more night races at Toscana.
Mount Aso is alright, but it's too hilly, as I prefer making tracks that are half-way realistic and realistically people aren't going to be building excessively hilly/rampy race tracks. Eifel kart is just the same location as Eifel circuit but with smaller, tighter tracks that don't give me enough room. I would like to create courses at the Tokyo Bay setting, but again they're too small and cramped for cars and I don't like to make tracks just for karts. I don't enjoy driving around Liege and both Liege and Alaska bore me scenically. They really wasted some potential for Alaska, as Alaska has a lot of potential for beautiful environments and nifty stuff like seeing the aurora at night, all of which was lost to GT5 which settled for probably the most lifeless environment ever raced in any major current-gen racing game.
Alaska:
Alaska in GT5:
That nifty reflection of sunlight on the snow is the prettiest you can make this track appear. If you make it daytime, you don't even get that and settle for just white snow and some generic, gray mountains in the background. If you make it snow, all you see is white on gray with no detail in anything anywhere.