64's Rainbow Road was so boring to me. No danger of falling off, there's that long stretch of straight road, and generally nothing happens for how long the track is.
I'm so fucking tired that I thought the topic was asking about Reading Rainbow.
I don't care if it has rails and is incredibly long... MK64 for that opening hill, and the option to jump off for a shortcut.
the MK8 version sucks, way too freaking short.