Edsger Dijkstra made an important distinction long ago; viz.
"Computing Science" is the formal branch of mathematics which deals with reasoning/algorithms/structures/models etc. "Computer Science" is the application of the above to practical technology needs.
If you accept this distinction, then of course the answer is Physics.
If you don't then the question itself is meaningless since we have only one "Theory" (i.e. CS) realized in only one "Physics Model" (i.e. there is only one observable universe). They are two sides of the same coin.