I never paid much attention to the track layout of the Rainbow Roads, I just accepted that they're the longest and hardest tracks of each Mario Kart game. And if the SNES and N64 versions were any indication, it means they were not my favorite tracks.
For appearance... N64 version looked okay (love the neon Yoshi head), but it just lacked that certain magic. Double Dash version just looks nauseating. DS version looks somewhat interesting, although it looks more like an F-Zero track. Going with the SNES version for appearance. There's just something about seeing all those colored tiles fly by.
Instead, it's the music that makes me determine which Rainbow Road I like best, and here I have to go with the SNES version. The later versions sound too happy and cutesy. The SNES one almost sounded majestic by comparison.
*looks up YouTube video of SNES version* Er... man, the SNES version looks simple... and only 22 seconds to finish one lap? Can't have it all, I guess.