Although I can't speak for the quality of Mario Kart 7's retro tracks as I've never played them, I know the selection of courses is great. Waluigi Pinball, Airship Fortress, Coconut Mall, Maple Treeway, Koopa Cape, Daisy Cruiser, the list goes on and on... So many good tracks were brought back to that game.
In terms of quality, of course Mario Kart 8 has the best retro tracks. All of the retro tracks in that game are beautiful and play smoothly. I wish they had saved some of MK7's retro tracks for MK8 instead. HD would have suited them nicely.