Coinbase

Coinbase offer USD to BTC trading and have publicly stated their plans to add other pairs as and when banking arrangements in other jurisdictions are secured. With the seamless integration of wallet and exchange features coinbase has rapidly increased its trading volume and provides decent liquidity on low spreads. 

Adding their wallet and suite of apps for most mobile devices Coinbase is rapidly becoming an industry leader. The design and ease of use of their trading interface is top of the range.