Not sure of anyone went to more than one event -- aside from organizers obviously -- hence question can be directed to them. But event was the biggest success in terms of audience, topics, presenters?
Hard to answer, because who other than the organizers (or really just Joel as I only went to 4 myself) has been to every single DevDays?
That said, I'd go with London, just based on size. It had 900+ attendees, which is almost 2x as large as the next one, which I believe was Toronto with 500. You need a fairly large group to get the network effects going full swing.