Monero

Technical specialists will say better, but I can tell you that Monero is the most popular one for anonymity

Keep in mind not all "privacy coins" are private. Some claim to be private because they have a privacy mode or just simply use IP obfuscation.

All monero transactions are private. The sender, reciever and amount are hidden. This is a true privacy coin because of these three features.

ZCash privacy is opt in

Dash I do not consider to be a privacy coin but is called a privacy coin since it has a privacy send mode.

There is Firo which I have not used much but I believe its privacy is opt in

Zano is a privacy coin that looks promising.

Mainly opt-in privacy versus privacy by default. With other coins they are opt-in privacy and by being opt-in most people don't use the privacy benefits and those who do stick out from the rest of the crowd. Wherein Monero all transactions are private no matter what and so nobody sticks out for having a private transaction

Zcash's privacy is opt in (bad), it has a CEO and not that decentralized at all. There are other facts, but these are the ones I could think of.

Objectively, ZCash has the equivalent of FCMP++ right now but there are a few issues. Firstly, most ZCash transactions are public so the anonymity set for ZCash is small and even smaller when compared with Monero which has a lot more users. Also, because of the former issue, every time you switch from public to private transactions, it taints your wallet just as it does with coinjoins in Bitcoin. There's also a much smaller purely private ecosystem (e.g. Haveno, Atomic Swaps, Serai, Tari, DarkFI, etc), since there is less pressure to do things privately. That makes it easier for ZCash to cave to governments request, even if it didn't have a company to target. It also lacks other features like Dandelion++ to help anonymity or P2Pool and RandomX to help avoid centralisation. Pirate chain, being a fork of ZCash, solves some of these issues by getting rid of the public blockchain but it is smaller and dependent on ZCash for new research so it is not viable yet.

Objectively, if used correctly, ZCash has better transaction privacy, at least until FCMP++ goes live with (probably) the next Monero hardfork. After FCMP++ goes live, no, Monero will be the objectively best privacy coin in all aspects.

In reality you have about 400 fully private ZCash transaction per day and 40.000 for Monero. You might literally be the only person making a Zcash transaction that day within a +100 mile radius, not much of a crowd to hide in.

There are many other factors like ZCassh having a CEO, dev fee, moonboy community and even their garbage tier mascot that make people prefer Monero.