Maple Treeway. It clearly made in return in Mario Kart 7 for a reason. I like Rainbow road as well but I feel like the Wii one was trying to be too flashy with all the crazy jumps and halfpipes, tricks etc.
As someone who has played MK Wii a ton, I’m going to say Rainbow Road. It truly tests your skills and it is the best-looking rainbow road of the MK series. You truly know how to play the game if you can do an entire race on it without falling off! lol
I really liked the maple tree way but nothing beats the rainbow roads I really liked it and it even was kind of a small challenge when playing on the Wii. It got me a lot of online wins