Poloniex has the highest volume, that's why it disables most of the withdrawal requests from most of the cryptocurrencies.

Poloniex has horrid customer support, and other than having nice looking graphs and a low fee they are not that great of an exchange lately. They received one of my deposits and never actually deposited it. Two tickets were ignored and months later still nothing. That wasn't the only issues I had with them just the one that finally made me decide to stop using them and that was several months ago.