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.
Mario Kart 8 managed to reinvent the retro courses into something that feels brand new, which makes it the best one. The only ones that didn't feel like a different course are those from the 3DS (Music Park, Piranha Plant Slide, New Bowser City), since they appear to be the same, but other than that, every course is like a new location.
Mario Kart Tour might be all right in this department but it remains to be seen how the classification for the Mario Kart 8 courses are named. If they call them after the console of origin (example: Wii U Super Bell Subway), it would be #1.
Mario Kart 7 definitely had the best selection of retro tracks, but Mario Kart 8/DX easily had the best revamps of retro tracks. They all had huge graphical and track redesigns, yet they still felt like the good old courses we know (well, maybe except N64 Rainbow Road and GBA Cheese Land, but those tracks are amazing anyway) and they are a blast to drive on.