What is the most challenging course you've ever taken?

Jumpmaster was tougher for me. 

USAF pilot training. Required willingness to study hard and memorize well. Physical demands were not exceptional, but stamina was very important. A resilient personality and body were important. Good mind-body coordination makes flying easier. Those that failed most often did so due to a lack of self discipline.

11C BNCOC. Then at 36 The special forces qualification course.

Infantry Plt Sgt Course.

Sapper School

I agree with SFC Andrew Messick. Cyber Awareness Training. Second hardest was Electrical Motor Rewind School in the Navy. That was pretty freaking hard.

JM School.

Master fitness trainer course- misleading failure three times a day for 6 weeks

Pathfinder School

Anyone's first selection/indoc course