I personally use bittrex the most. I think it is very easy to use, has a lot of activity making it easy to buy and sell, as well as i like that I get notifications upon every login and have to approve my withdrawals via email. It also has not been hacked or had any major issues.
Wow! I am getting one support for Bittrex. Though you have not said anything about others, should I assume that you are not trading with the others?
Yes, I have used the others and was not impressed or stayed away because of security flaws. I now only use bittrex or internal markets.