Is it better to have a negative clock skew? Why? If we compare it with a positive clock skew, which is better?
It's not necessarily better, in fact, it depends on which timing is harder to respect. Quoting Wikipedia, "Positive clock skews are good for fixing setup violations, but can cause hold violations. Negative clock skew can guard against a hold violation, but can cause a setup violation."
The best practice is to design circuits that work well with zero skew. This makes designs much less prone to failures caused by later modifications.