I'll go with Monero. It is widely used by merchants, not only in the darknet markets but also in the clear net as more and more every day are concerned about privacy and anonymity and Monero offers the best of both worlds.

Zcash offers transactions like Bitcoin, which is a big advantage over Monero in terms transparency that some people require in the cryptosphere (exchanges, for instance), while in Monero, all transactions are hidden. Still, when it comes to privacy, Monero is top of the line.

It's worth mentioning that there's currently some rumours that Zcash could be added to Coinbase, though there are only rumours and speculations, this is creating a bit of hype given how everytime a crypto gets added to exchanges, it gets pumped a lot. Might be a good time to get some Zcash now that its price has gone down.