I've been using http://www.kraken.com of late and it's been great - no problem wiring money to/from my bank account, easy to convert between LTC and BTC and market orders close immediately.

I've only been trading in small BTC amounts though so not sure if you'd hit liquidity problems if trying to trade large amounts (kraken's volume is quite a lot smaller than the bigger established players like bitstamp)

https://www.bitstamp.net/ ?

I started using bitfinex recently. Other exchanges I've used have had miriads of downtime, and I just moved off campbx because their site is so slow (and their market cap has shrunk huge). The other plus is that their fees are often half of other exchanges, I went from a .5% comission on cbx for sales to a .15%.

Bitstamp is also good, I hear, but I just went with finex because their fees are lower =P.

I think it is important to use an exchange outside the US though, because I expect the fed to start going Orwell on any bitcoin company soon.

I've done most of my trading on https://btc-e.com/. Never encountered any problems. To my knowledge it is also the second largest exchange for BTC/USD by volume ( see: http://bitcoincharts.com/markets/currency/USD.html - MtGox' volume just jumped a lot in the past days).

Bitstamp seems to be good.

There's also the Canadian https://www.vaultofsatoshi.com/, which seems to be top notch as far as the tech (multiple 2FA options, OpenPGP cryptography for email, etc.).

Coinbase.com is a great one, and they recently received venture funding