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 was the time when poloniex was one of the top crypto exchanges with the high volumes but the time never remains the same, poloniex is now not in top 50's because of high fee and low volume, I would suggest you to use bittrex because it's interface is very user friendly.