You should honestly have every retro course in the poll.
Anyway, while Yoshi Valley is my favorite retro course it's not necessarily my favorite remake. My favorite remake of a track from a prior Mario Kart game in Mario Kart 8 and possibly any Mario Kart is probably GBA Ribbon Road. I kind of thought of it having a toy theme before its Mario Kart 8 version was shown, but I was expecting something like it taking place in the Toy Time Galaxy rather than (likely) a child's bedroom. The remake of Tick Tock Clock is also really great, and GBA Cheese Land's pretty good too despite it no longer taking place in space.