I had recently started my own Super Mario Maker 2 course sharing thread featuring 19 (of the now 20) courses that I had created so far before finding this old level sharing thread. I'll post the Course ID and Description of my most popular Course according to # of players and amount of Likes in case anyone still views this thread and is interested in reviving it.
Mario Plays NES Games
Course ID: TW6-PMX-VQF
Description: As Mario, go home to your living room for an afternoon of playing three NES classic games on your NES. Turn the NES on and hop inside your TV to play a level of "Mario Bros.", "Donkey Kong Jr.", and "Super Mario Bros."
I'll also be checking out the courses that have already been posted here in the meantime.