The best answer I've seen, is that zero knowledge proofs as Zcash uses in isolation are superior to the ring signatures that Monero uses, but Zcash has one major flaw in that it allows its anonymity pool to be small.
What I mean by this is that transactions in Monero are always private, while users of Zcash can choose to send public transactions. What this does is it makes the group of users who send private transactions through Zcash much smaller and far, far more susceptible to being identified through metadata and process of elimination.
This is heavily simplified - and Monero community members would be happy to get into the weeds with you about it if you were to visit their realms on the web, I'm sure.
What I mean by this is that transactions in Monero are always private, while users of Zcash can choose to send public transactions. What this does is it makes the group of users who send private transactions through Zcash much smaller and far, far more susceptible to being identified through metadata and process of elimination.
This is heavily simplified - and Monero community members would be happy to get into the weeds with you about it if you were to visit their realms on the web, I'm sure.