One of our most difficult; few will solve this 18 pc 6x6x6 burr. Is a challenge to even take apart..  Requires 2 moves before  you can remove the first piece in the dissasembly 

Substantially sized at approx 4" cube.  

If you're looking for the really tough ones, along with this try Dragon Burr, King Snake 4x4x4, Ziggurat, Double Revlving Century,  Ramube Octahedron, or Shippers Dilemma Z.