Well, I will always recommend that you use Bittrex, the echange which I use. Bittrex very rarely encounters any problems and very rarely locks any account- unlike Poloniex.
Poloniex is a big mess. It is too large for the devs to handle and they have not taken proper steps to improve it.
However, while I recommend that you use Bittrex, if you are looking for an exchange which supports a lot of currencies, then I would recommend YoBit. YoBit is your best option for a very featured exchanged with a fair amount of cash flow, but I only recommend YoBit under specific circumstances (If you can't use Bittrex)!
Good luck and stay safe!