mario party superstars for sure! i've always adored the mario party franchise because it's so much fun, especially with friends. the boards, the minigames, everything about it is fun! watching the mario party streams and watching everything just go to chaos once the game really gets started is just too funny
mario kart 8 is really fun to watch too, although i really don't care for this specific mario kart (double dash, ds, wii and 7 will forever stay close to my heart ), it's still pretty fun!
i dont understand a thing about smash or splatoon (smash, i'm beginning to understand but splatoon? nope.) so it's pretty low on the list for me.
Having watched at least one stream of all four categories, I choose Smash. That's the only game where I'm not completely lost on what's happening, despite not having played the game. It's also a little bit easier for me to distinguish if an interesting move happened. It also has Fire Emblem characters in it, so that's another plus for me.
Mario Kart is a close second. It was also fun watching the participates dodge bombs on the map with the Thwomps.
Like the other poll, I threw the mystery game (Among Us) and an "I didn't watch anything" option onto the poll! kinda wish I could remove the word "in" after "spectate", but alas