Rainbow Road has never been my favorite course in any Mario Kart. It's not because it's typically the most challenging track to race on: I like to have a good challenge driving and I've played tougher racing games with pretty hard tracks (Burning Rangers on S difficulty in Sonic & All Stars Transformed anyone?). No, it's because I find its aesthetics boring. All of its BGM sound practically the same. In contrast, Bowser's Castle had atmospheric scary tunes, threatening tunes, and just outright hammy baddy tunes that mix it up. Rainbow Road is pretty much: it's happy, it's in space, it's ethereal, for practically all times it appears in. Not to mention, it's always just a random floating road in space. Mario Kart 7 at least tried mixing it up and 8 gave us an artificial Rainbow Road which was a nice change of pace but neither were my favorite tracks in Special Cup regardless (7 had DK Jungle and Rosalina's Ice World while 8 had Bowser's Castle).
I'd like to see other takes on Rainbow Roads for once. Have a Rainbow Road set in a dream world in the day time, sorta like Rainbow Coaster and Rainbow Downhill from Mario Kart Arcade GP except wackier and perhaps even psychedelic at points. Make a Rainbow Road be like a party house with rainbow tiles and a giant disco ball. Maybe even make it Christmas themed. Anything for a change of pace.
BTW, I do realize what I said also applies to the circuits and yeah, I do agree that I don't care for most of them too. They tend to be extremely generic, boring tracks, just with a Mario character slapped on it. Luigi and Mario Circuits tend to be just bland, while any other character that gets it is bleh too. I get it that they're supposed to be the beginner tracks but damn, Wii Luigi Circuit and Wii U Mario Circuit is so boring to race on, I'd just rather race on Mario Kart Stadium or Yoshi Park 1 from Mario Kart Arcade GP 2 any day of the week.