MK7 may have the best online (for you; I thought MKW was better for joining friends) but MKDD had LAN. And being in the same room is way more awesome than online gaming. (it was new back then; handheld LAN sessions are too common now ;))
DD also felt very polished (Wii and 7 felt a little beta imo, Wii even having weird menus and remastered music) and really good karters were able to pull away from the lot. That way tournaments were lots of fun because you could really see who'd make it through to the finals and learn something. In Wii most players can keep going with the good ones if they don't fail too hard. The items in DD were balanced in a better way and the tracks were more fun. The game felt faster (due to wideness of the tracks etc.) but controlable.
In the end, I don't see any difference between "best" and "most favourite". The best is what people like the most. If someone loves playing online they will pick Wii or 7. If someone loves retro/2D they will pick SMK and say MKSC is cool as well. etc.
7 for me was just picking the right kart and wheels, learning the glider tricks, staying behind on 2nd or 3rd and winning overall. Pretty boring.
