ctgp is by far the most important mkwii mod to have! it adds a whole bunch of custom tracks and retro track remakes, on top of various quality of life improvements. you can also play custom tracks online.
retro rewind is kinda like ctgp except all the custom tracks it adds are tracks from older/newer mario kart games. it runs on wiilink servers (ctgp runs on wiimmfi) which are supposed to be better. other features are added like more options/game modes for friend rooms as well as differnt normal options like showing everyones times after a race ends online and all vehicle inside drift or outside drift.