For regs I'd probably say either Moonview Highway or maple Treeway. The bumpiness of Treeway is annoying and the hotboxes on the trucks are stupid.
For ct's either melting magma melee or peaks. MMM for the bumpy road and obstacles, they just make it so uncomfortable to maneuver. Peaks for the physics of the mushrooms. You'll fly through them half the time or just get the wrong bounce off of them. And the respawns are so unfair.
My guy @Paris is a beast though. MMM is scared of him!
Just a personal thing, nothing stays hard if you learn it, obviously.
I've had tracks I thought were extremely difficult like Melting Magma Melee, but hey I'm 12 worldwide on that track now, I've grinded and master it now.
For normal MKW, I say Moonview Highway and Rainbow Road. The tracks are fun and wonderful but certainly hard to maneuver
For CTGP, Mushroom Peaks and Melting Magma Melee are the ones that give me the most trouble. The mushroom paths in Peaks are what makes it so complicated for me. And MMM, well, that entire course is an adventure to survive
Nonetheless, the 4 are a ton of fun.
Sherbet land and Shy guy beach suck! For real though the hardest track is moonview because you need to learn all the car patterns and backspamming can be pretty tricky to avoid. For cts, no tracks are hard for me, I figured out the mushroom section for peaks so no tracks imo.