I think it is strange question as best crypto coin is bitcoin for sure, no other coin is popular like bitcoin, and there are many different good trading sites so hard to pick best
Obviously bitcoin is the best crypto currency but we need to check for second and third best to trade against bitcoins. Because, trading bitcoins against fiats is not that much profitable comparing to trading bitcoin against other crypto currencies.
Almost all trading exchanges are similar, but they are having some unique feature with them. Yobit and poloniex are good in my experience.
actually people should have their own research to determine which one altcoin that they will traded ,
we can not say this 'A" altcoin is the best to trade rather than the 'B' altcoin ,
if we do trade the same altcoin at same time and same way , what will happened to market?
also so far poloniex and bittrex leading in the altcoin trading exchange , they have a lot of user and backed by large bitcoin in their cold storage.