"Hardest" Logic Puzzle You've Found?

[rush1169]

So, on this site I hit "refresh" until I get to a puzzle that has a <30% success rate. I'm pretty sure I've done all of them and the challenge is gone. . .so, I was wondering if any of you know of any other logic sites that may present harder challenges? Maybe 6x6 or larger grids or clues that require more 'logical pathways' to be examined? If so, please share.

The 'hardest' puzzle I've found is here: http://www.puzzlersparadise.com/onli...Deliveries.htm

But surely there are more?