- Bittrex
- Bitfinex
- Livecoin
- Cryptopia
- Novaexchange
- Yobit
Which exchanger, you should use, depends on the altcoins, you want to trade.
My favor is Bittrex, because of the huge varity of different cryptos.
Cryptopia has nice additional features with freebies on chatbox.
Also yobit pays free coins.
Livecoin has interesting established and new coins (ICO)