I'd say that Stamina Master is probably the most frustrating level in the entire series.
I've literally played it for several hours straight and wasted well over a hundred tries -- and I still haven't been able to legitimately complete it. I can complete the game's stages pretty easily (even some Master levels, and I've done all of Expert without dying once), but this level frustrates me to no end.
What do you guys think would be the most difficult Monkey Ball stage?
I started a account with sega JUST to talk about this **** game, i may not have chosen the right forum because all i'm going to do is talk mad trash about this addition to the series, but thats for another thread, to you i must say, there isn't a single DIFFICULT level in step and roll, however there are a few retardedly irritating levels which are only time consuming because there's crap in your way, not because the board itself is complex or challenging. This game was designed for the stupid pad, it's bad enough they had to drop the difficulty of boards for the motion sensor in the wii controls, but now they had to do it again for the stupid board. Frigging groves on half the levels (that are un-necessary and irritating if you don't have a wii board, and probably still for experienced SMBers) and slightly less irritating versions of the bumper EVERYWHERE (to try and make up for the loss of strategic thinking and intensity of the boards ) do not make for a good SMB.
To actually reply to your initial post, for me, 8 rings was the hardest level ever in all of monkey ball, i never finished Adventure tho (you have to get every **** banana in all the worlds, not easy unless you have a map of all of them and know where you've been) When i saw a youtube video like years later (i don't even think youtube existed when i first played that level) that showed that level and how you can skip right through them i was mortified, i spent so much time dealing with that level, the fing camera sometimes really makes things really hard. And i don't remember SMB1,2, delux adventure, like almost at all, but i remember that frigging level.
Also i remember in 1 or 2, theres a level where you can choose your exit, easy med or hard, and the hard one is a pin point walk way for a good length, that i never got.