Out of the millennium prize problems, what one do you think is the most interesting/important?
Why not? I think the existence of turbulence on multiple scales is neat. Other interesting questions that arise from it is the idea of deterministic chaos.I hope stokes doesnt get any strokes from this (well he can't, he must be dead), but i voted for RH, although possibly because it's the most popular one.
The mathematics which described Fermat's last theorem looked simple but yet the math used to solve it was not simple.One does this by looking at what one has to do and the level of this mathematics. For example, hodge conjecture is from algebraic geometry, riemann hypothesis is from number theory, Yang-mills is a combination of quite alot and some of the mathematics does not exist, so that might rank more difficult because there is a very vague starting point.