The Universal Turing Machine. Because it is universal.
Not only as the theoretical archetype for digital computing as we practice it today, but as a least common denominator — translating between sequence in time and pattern in space — that lies at the foundations of mathematics and suggests the possibilities of a communications medium we have only just begun to explore.
Life and intelligence that achieves widespread distribution across the cosmos (and over time) may be expected to assume a digital representation, at least in some phases of the life cycle, to facilitate electromagnetic transmission, cross-platform compatibility, and long-term storage. This requires a local substrate. And we are doing our best, thanks to the proliferation of our current instantiation of the UTM (known as the PC) to help. When we establish contact with such an intelligence, will we receive instructions for building a machine to upload Jodie Foster? Probably not. The download will proceed the other way. To paraphrase Marvin Minsky: "Instead of sending a picture of a cat, there is one area in which they can send the cat itself."