By the currently accepted definition of Computer Science (at the nexus of its core basis in mathematics and philosophy), I would say “On Computable Numbers, with an Application to the Entscheidungsproblem” by Alan Turing, without any close peer to date. It is the first paper that invoked the most all-encompassing idea of a computer (e.g. not necessarily having to be a physically realizable electronic computer, which one could consider more of a matter for engineering).

Its contents have had continued relevance as computer scientists study the properties and capabilities of non-deterministic computers.