I'd have to say that my most productive spot is Starbucks. It probably beats my home office because I don't do it that often and the change of venue is extremely refreshing for me. Even just leaving the home office from time to time and working out in the front courtyard changes my productivity level.
Of course, if you have to conduct a conference call with a client, then the home office likely wins - it does for me at least. Walking around outside with a bluetooth headset on is fine for quick calls, but wind, noise and other issues can come up. There's nothing like going into the office and shutting the door - it gives me the ability to focus 100% on what I'm discussing with the client.
If you're an independent consultant, where's your favorite venue away from your home office?