The way I actually determine difficulty is from a program I wrote that solves the puzzles. The program implements a number of techniques to solve the puzzles and determines how hard it is based on the techniques that must be used and how quickly it can be solved. I hope to have the solver hosted where you can get to it soon, so stay tuned.
In any case, here is the hardest puzzle I have come across.
And another difficult one.
Give them a try!