Which norm is the best to match descriptors?

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it depends not on the images, but on the descriptors you use.

for binary descriptors, like ORB,BRIEF,BRISK you must use the HAMMING norm, the descriptors are bitstrings, not numbers

for float descriptors, like SURF or SIFT, use any of L1, L2, L2sqr (L2 probably works best)

the Feature2D class also has a defaultNorm() member, which you can query.