Silicon Valley Bank, if you can get an account with them. Not sure about their online service, but the operation (which is obviously much smaller than the US one) is pretty slick, runs great events for startups and is generally supportive of startups and the particular difficulties they have. So - probably worth checking out.
In terms of what to look for - integratable bank feeds to sync with Xero/FreeAgent/accounting software of your choice (most UK banks do it now, not sure about the US), and transparency of pricing - I discovered, by trawling through the small print that the bank I was about to go with was going to charge me £0.34 for EVERY incoming transaction. Not a great option for a subscription based system dealing with ~£3 payments. It was incredibly difficult to find.
