So it seems Monero is the best out of three, right ?
For the moment, I think so too. However, ZCASH has very interesting crypto. The main problems I have with ZCASH are:
1) the post-premine by the company
2) the trusted setup
3) (not sure but I think) that ZCASH doesn't use compulsory private transactions, like monero does.
That said, ZCASH DOES have very good ideas. I think that the main fundamental problem, the trusted setup, can be vastly improved. When that will be the case, when it will be a grass-roots chain (no postpremine) and when the private transactions are the only possible, ZCASH has better anonymity properties than monero. But without these 3 aspects, I think I prefer monero's anonymity, although it is not total.
DASH has, IMO, "historical" merit, in that it is one of the first cryptos trying to implement what could improve anonymity. I think that DASH's anonymity tech (mixers) is outdated, and requires a lot of extra structure (master mixer nodes...) that has been made obsolete by the technology of ring signatures (Monero) and also by the technology of zero knowledge proofs (zcash).