The best until now for me is Bittrex.com, this exchange site has many good altcoin listed on there and a lot of people (including whales) trading on bittrex everyday.
The second is, Poloniex, on this site we will see only super and elite altcoins listed on their site. But unfortunatelly many people dissapointed with Poloniex's support.
Third. Liqui, cryptopia, and coinexchange. Never had any issue with those exchange sites, but the volume on those site are lower than Poloniex and bittrex has.