How to know the best GPU for mining zec, eth and other altcoin

I have being reading alot of post on mining, but i have not gotten what am looking for.

What GPU specification (memory, clock speed, etc ) determine the speed for mining
and how will i know the best.

I dont care about the power consumption.
i just want to know and get the best hashrate for mining.

I will like to learn from you all on this topic