Definitely going to be a controversial choice... 28th goes to
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Toad's Factory
Okay, let me start by saying that objectively this track doesn't deserve to be in the bottom 5. There are definitely other tracks that are less original, but this is a subjective ranking and I'd be lying if I said I didn't really dislike this track lol. Something about this track has always rubbed me the wrong way and it's really the only track in the game that I wouldn't be happy to play. I will give it credit in that the concept and design of this track has some originality. The cargo area over the water in particular is quite nice. But for whatever reason, this track as a whole has always bored me to tears. Despite the track being very busy, it track still manages to feel very static and one dimensional to me. Additionally, the construction/mud section at the end is just a chore to play. So although the track is not necessarily designed poorly, I just don't have fun playing it. I don't think I can rank it any higher because I will at least tolerate pretty much every other course. I can understand the appeal, it's just not for me.
Disclaimer: This is a subjective ranking based purely on how much I personally enjoy the tracks rather than using any set/objective criteria