In my opinion the best trading exchange for day trading is that which have high numbers of tge users because thier the trade speed remains always speedy and we can make earnings with the small increasing and decreasing earnings .
So it is a better option for us to go at those exchange which have better potential of the users .
Personally here i would like to suggest you poloniex ( since few people facing problem of deposit for one week but now I think problem are solved ) , Bitrex , bitfinex etc These are those exchange where you can found better better high profile coin to make trade and trade speed will remain much high according to demand .