We know that Bittrex and Poloniex are the most sought after exchangers.
But I want to know which one is best to trade? and I want to know why.
Thanks.

There are many exchanges in currency world or currency trading system. Bianance, Hitbtc, Bittrex, poliniex, cryptopia, Etherdelta, IDEX. Above all exchanges I like Bittrex and poliniex both.
I like bittrex and poloniex too and so far, I don't have any problem with both exchanges and I still trade with both sites. but I see many people have a different problem with bittrex and poloniex and maybe they miss something so they can get any problem. and if they can be patient for a while and follow what bittrex and poloniex wants, I am sure that their problem will be solved.