Coinbase is the popular opinion, fairly unanimously. You can only Instant Buy in the US. As terrible as waiting four business days is, they do it to protect themselves from user scamming or having insufficient funds. But this slowness is really due to bank transfers... unless a company can take the risk, they need to pass the waiting period on to the customer. So that's what CoinBase does unless you have a credit card as a backup and don't buy too much.

Mt Gox is troublesome if you're in the US (probably other countries too) simply due to international currency issues. Hell, I still haven't gotten my identity verified after three weeks.