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.
Your best bet is poloniex.
They offer margin trading and are one of the most trusted sites in the crypto world. They also have the best volumes out of all altcoin trading platforms, and competitors are only big companies such as BitFinex and Bitstamp.
However, they only support around 10 coins for margin trading.
You can't really short any shitcoins that are obvious that they are going to crash, aka bitcoin plus/bitcoindark.