Who told you that Eth and Zec are the most profitable coins to mine?
The following website claims that ETH is the most profitable:
http://whattomine.comAssuming that you have put your hands on RX 580, which is also the most cost effective (price, hash rate, power consumption, etc...)
If you are speculating about future gains of other altcoins then this is not guaranteed
But for GPU mining ETH is definitely the most profitable as for now