The sixth sick sheik's sixth sheep's sick.
I didn't know this one! Nice :) All the /ʃ/ and [θ] sounds combined with [s] make it very difficult to pronounce.
It is easier to pronounce than Suchą szosą Sasza szedł.
In "Suchą szosą (...)" you only have [s] and [ʃ] combinations that are problematic. In Milo's twister there is also [θ] (which doesn't have a Polish equivalent) - for a Polish speaker it's definitely more difficult.