He and Church made the most intuitive models of computation, and his model was able to directly tackle the halting problem / restatement of Gödel's incompleteness in computation.
Church's work is very important for helping us reason completely formally using lambda calculus and setting up the longstanding relationship with language theory, but Turing's formulation gave a grounded equivalent model that admitted an easy understanding of what an algorithm actually is. Now, the Church-Turing thesis is basically an effective definition of an algorithm, but Turing's tied the mathematics to the mechanism, seen in the von Neumann / Harvard architecture. That, and his notion of the stored program had huge effects on the way we think about practical computer systems (and how we reason about instructions as data themselves).
By no means is he the only big boy in early CS, nor is he even the only father of CS, but his contributions are noteworthy. I wish we talked more about Karp, Cook, Rabin, Lamport, Levin, etc. though