Decentralized exchange is my first choice,because of security concern and withdrawals fees.Although these are centralized but my favorite exchanges are
1.Binance ( it’s not yet decentralized. But it plans to go decentralized in the future and it’s feature easy to use)
2.Bittrex (The largest cryptocurrency exchanges because of its high trading volume)
3.Hitbtc
Among my favorite decentralized exchanges are..
1.Etherdelta
2.Bancor Protocol