I forgot to mention heavy Mii is also a viable character.
I forget the exact name of the best karts but the tires are usually Slick, Crimson, and the default.
Mach 8 kart, Pipe Frame, and one other kart are used mostly.
ATV's ONLY usefulness in a speedrun is Shebert Land, which is a shame, since I do like ATV's.
ATVs are also useful on the ice sections of Mount Wario, and their bouncyness is really handy on the ski slalom and ski moguls sections. I also do particularly well with them in Piranha Plant Raceway (or whatever it's called, one of the 3DS levels - I often mix up names since some course have different names in different regions, so I'm playing a PAL copy, but often watching videos of NTSC copies).
Except the Koopaling thing lasted a page, this has been lasting for over a month on and off lol.
Now you're just splitting hairs, because many topics have been on and off in this thread (including the Koopaling thing).
BTW, I don't have a problem with discussing favourite Koopalings or anything, I'm just using it as an example - if you're gonna complain about it when people want to talk about Rosalina in some form, you're obligated to complain about the Koopalings, or heck, Luigi being brought up, or any other character. Otherwise, you're being selective and biased.
If you really don't want to have Rosalina pop-up in discussion at all, you're free to make a "NO ROSALINAS ALLOWED >: (" version of this topic or something.