There isn't any at the moment. All of them are having problems. If someone ever says to you "exchange A is great and nobody ever had any problems", he is lying.

Polo: Many withdrawal problems. Locked currencies, wallets...
Cex.io: Is in trouble with the FEDs
Btc-e: same
Kraken: same
Finex: Ticking bomb
Coinbase: Joking right?

It is time for us to push for decentralized exchanges. Oldschool exchanges has little time left.