When I speak with people about what we use, LeanKit, they are often concerned with tracking bugs via other trackers, like JIRA. LeanKit has an option to number your cards with a JIRA number and then providing a prefix URL that appends the tracking number to link to your tracker. Usually they are then more concerned with using more than one tracking tool.
We use LeanKit pro features, such as tagging multiple developers on stories and the class of service flags. Class of service flags allow us to tag stories with the target customer within a product.
Not saying LeanKit is the best, but it's what we use. A few years ago, when we decided to switch boards, a few stuck with Agile Zen, but most products switched to LK because of the $5 cost per user, as stated before.
The CEO of LK has been really responsive with our needs for the service too. I actually have had several phone calls and a video tutorial by the CEO of LK when I couldn't get it to do some things I wanted. He showed me some alternative ways and also incorporated some features for us. They also allowed us to trial the Pro version for free until the features were out of Beta.