Poloniex is the most popular altcoin exchange, has a smooth and beautiful interface and a huge volume with good liquidity.
Unfortunately the support is very slow to reply (sometimes I waited for more than a week to get a solution) and their withdrawal system is broken (it’s not rare to request a withdrawal and see the withdrawal stuck as “Pending…” for hours (if the withdrawal doesn’t result in a error). I wonder if they do this on purpose to discourage people from withdrawing or the withdrawal code is full of bugs.