Robin Milner was indeed one of a kind. One of the Great Masters of Computing, for sure. I'm currently taking a class on models for Concurrent Computing, and the first subject is Milner's CCS: http://en.wikipedia.org/wiki/Calculus_of_communicating_syste...
The way he thinks about processes, communication and synchronization is just beautiful. When I was able to "prove" that the implementation of a two-bit buffer as a chain of two one-bit buffers actually behaves as specified, it was as enlightening as when I first saw a higher order function in Scheme.