Which ARM based hardware is best for running ROS?

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I think there is no single answer for this question. This depend on which processes you want to run (attaching sensors, doing hardware interfacing, pointcloud processing, SLAM mapping, ...). ROS is only the framework and doesn't tell you the system load. You should be able to install it pretty well on all ARM hardware that gives Ubuntu support and preferably has a network connection. Look at EROS for support.