Im not asking for things which are repetitive, like an infnite sybil( just switch grips to put packet in left hand and redo it alot) or something, but one which has different phases and lots of them. something like the Jackson 5?
Most difficult is actually the simple ones, such as the Padiddle, Carnahan fan, or under pressure, mostly paddidle though. Generally the most difficult thing is style and smoothness, which take lots of time and effort, whereas learning many flourishes in the first place is often the easy part (greatly depending on how many you have already learned, and amount of experience, after about 5 unique ones, similar ones are fairly easy to pick up, and it becomes harder to find a challenge).
Longest is maybe Jackson 5. I also made one that's a minute long called Complicatrix that's based on the dynamo shuffle. Complicatrix is also difficult because it's complicated and long, but I'm not sure about relative difficulty, though I made it to challenge myself.