Since the NES controller actually has the buttons in the order B, A...
...it's best if Z = B, and X = A.
This also matches what Pico-8 does, with X being the primary/Yes/confirmation button (similar to NES's "A"), and Z (labeled as "O") is the secondary/No button. In Pico-8, you can also use N and M for secondary and primary respectively. This would be useful if people also allow WASD keys for movement -- which is ideal from my perspective.
My preference is also to have the Tab key used for Select. But I do like Enter for Start.