I've always had trouble with the SNES and Mario Kart Super Circuit tracks. In SNES, it was that some cases, I couldn't see what was going on, due to lack of map and oh, the game controlled...not that good, I'd say. Super Circuit's problem for me, lies in the awful controls.
I'd say 64 also has...
You know, seeing Superstar Saga with zero votes is a bit strange. I would have thought that it would have least got more votes than Partners In Time which on other forums seems to be known as the worst Mario RPG, bar Sticker Star...and Color Splash, whenever that comes out, you just know it's...
One of the hardest ones for me, personally, would have to be Don't Touch Anything from the Challenge Mode of New Super Mario Bros U. There's also Extra 5; Kamek's Revenge from Yoshi's Island. There are others that are really difficult but those two were the first to come to mind.