After trying out YoBit,I'd want to start marginal trading.The usual bitcoins to alt and vica versa.The site must be very supportive,less document requirements and supports most of the altcoins.
Also I will be day trading at times or invest in a stable alt for a week/month.My main concern is support,they trading website must support most of the alts and definitely bots.
I think bittrex and poloniex would be better for day trading, they has margin trading and they also had the most volume for altcoins. They are all reliable and always have support staff in chat.