Well
the SNES's Rainbow road was... annoying. I didn't really like it all that much. Though... there's something about the music that I like, though it's not as nice as the next games' songs.
the N64's brings back a lot of memories of me playing it and schooling my brother. It has really nice music, but the length and lack of variety in the track was rather annoying.
the GBA's music is... what I think anyway, some of the best I've heard in Mario Kart. It's not the same tune from the last two games remixed, it's a whole new tune. The level design, though didn't go uphill... at all... was really nice too, though the walls caused you to jump.
The GCNs is the best graphically, but that's to be expected. Track design was nice, though I'd die on the spiral because I'd try to drift and mess up, causing myself to launch right off...
The DS's was different from others, because it had a loop and... I think a corkscrew... I don't remember. Music was nice, though not as nice as other songs.
And the Arcade's, well, I've never played it. My brother has, and said it's nothing special. I don't know what to think, though.
Music wise, GBA is best. And then GCN. And then a tie between DS and N64. And then SNES
Level Design Wise- GCN, and then GBA, and then tie between DS N64, and SNES
Overall, I like the GBA best