Final answer:
Non-obviousness is the most difficult criteria to surmount for patenting.
Step-by-step explanation:
The most difficult criteria to surmount for patenting is non-obviousness. Non-obviousness requires that an invention is not simply a variation of existing technology and would not have been obvious to someone knowledgeable in the field. It requires evidence of unexpected results, a unique combination of elements, or a significant departure from the prior art.