SMK- Best rainbow road in terms of just how much it kills you. When you hit a jump wrong just a bit off and it overshoots you because of that Thwomp that was going to be in the way, land on the edge, hop frantically while trying to veer the other way, repeat, and barely make it but keep going, that's what made this the one most awesome one for me. And who can argue with the fresh-out-the-80's techno kinda electric bump song they had going on? Can't beat it! Also when you started to plateau on the power band and were cruising, you'd be getting around quick! ...or any track for that matter on SMK.
MK64- I liked the length, but on the second lap the advertised "2000m" wasn't appealing to me. Sure you can VEEEEEEEEEEEEEEEEEEEE like a motha around the floating star and in some other places, but other than that.... kinda bland, save for the neon faces in case you forget which character you're playing and never see the front of them. Good sense of speed. Fun to avoid Chain Chomps.
MK:SC- Fun, good music, Mario Party 2-3 voiceovers. Brings back the days.
MK:DD- Rainbow Road kinda seemed bland to me, also. Very annoying with drifting the 195º hairpins at times, but fun when you'd almost fall off but get it. It was cool that Mushroom City was under it. The pipe didn't bother me at all. How else would Nintendo elevate you quickly in a Mario game? A pipe, lolol.
MK:DS- Very good rainbow road. I loved DS altogether though.
MKWii- The long tradition of using that little bit of the MK64 rainbow road song was gone now, replaced by part of Good Egg Galaxy, which I doesn't bother me, really. I figure Nintendo was like 'no more 64 bits because we're revolution' Very fun track, elevating pipe gets replaced with Sling Star (with corresponding sound effect from Galaxy when you enter it and going up in pitch if people behind enter right behind you, or vice versa. Too bad it didn't go with the rainbow road music like galaxy's sling stars did. Slowest feel of any rainbow road. Unless you're boosting somewhere you're crawling and they tried to compensate for how slow it was gonna be by adding all those speed thingies.