“Collect evidence about your environment from various sources; update your model of reality based on evidence collected; act in accordance with what your model of reality indicates is best for achieving your goals; repeat continually forever” would be a great candidate for The One Sentence.
Related to: What Do We Mean By "Rationality?"
Rationality has many facets, both relatively simple and quite complex. As a result, it can often be hard to determine what aspects of rationality you should or shouldn't stress.
An extremely basic and abstract model of how rationality works might look a little something like this: