Dash is confident that it's coin mixing solution is anonymous enough and multiple rounds of mixing is good enough to thwart transaction analysis. But its focus is mainly on making itself the most usable coin with its instant transactions and a decentralized API to make it as easy to use as Paypal. its anonymity/fungibility is just one of its features.
Monero is confident that its anonymous by default and its ring signature combined with RingCT offers the best solution and that its ring signature size is strong enough to make it anonymous without relying on a trusted setup. As ring size (anonymity) is increased, it occupies a lot more space too. But it's a solid project and accepted by one of the largest darknet markets. GUI beta was released finally after 3 years of dev. note, it only recently gained significantly.
Zcoin boasts a larger anonymity set than Monero/Dash and is a lot easier to integrate to existing Bitcoin ecosystems since at its base it's still using Bitcoin. Technically also it's traceability set is bigger. Its approach also makes transaction graph analysis impossible since zero knowledge proofs breaks the link between old or new coins. But critics will say we use a trusted setup from the RSA factoring challenge (though we hope to remove this trusted setup at a later stage). Note that even if trusted setup is broken, anonymity is not broken, just that people can do zerocoin spend transactions without burning a coin but because supply is still auditable, this can be easily detected. We will also be the first to use the MTP protocol which is meant to be an improvement of the Equihash protocol used by Zcash. many other devs in the pipeline but focusing on the basics first. Rather underpromise and overdeliver. We feel that our solution offers the anonymity power of zero knowledge proofs without sacrificing too much on usability or integration.
Zcash boasts the theoretically biggest anonymity set. its use of zk-snarks allows the proofs to be very small (so you can fit a lot in the blockchain) and private transactions do not have any transaction value. Its anonymity set is larger than Zcoin as it encompasses all transactions in Zcash. However the trade off is that mobile integration i forsee is a problem, long computation times with high memory requirements (phones may take much longer), and the biggest one is that if there is an issue with the trusted setup or a vulnerability discovered, coins may be generated without people knowing there's a problem. Also currently i think if i'm not mistaken it's only available on Linux. So dev is a long way off in making it a usable coin.
So as you can see, to me there's no best coin. It depends on what you want. All good projects.
To me, the biggest risk is if Bitcoin itself implements some anonymity features which makes a lot of these anonymity coins for the regular guy obsolete since Bitcoin's anonymity might be good enough. For example BIP-47 would be a good step forward. But you know how things move with Bitcoin...
