Here's an extremely difficult tessellation that I recently created. Even though it was my own concept, I still needed a second attempt to actually fully execute the proper folding of it. 

The design began with a circular series of triangles. Their connections alternating between tucks and flagstone style collapses. See the crease pattern at the end of this post for further details.

That circle of triangles led me to discover the incredibly hard to collapse center portion. It's a small hex twist with upside down right triangles. They collapse into the next repetition. 

The photo of the reverse view shows pretty well how it works. It's small and cramped and has a lot of fiddly little folds. 

Thick and sturdy paper is a necessity. As is quite a lot of patience. 

It's advisable to work much more heavily on the back side. Getting those collapses to cooperate really is the key to successfully folding this  particular origami tessellation. 

Crease pattern is at the end.