I would define it as something like, "The course of action one would take if they had perfect knowledge." The only problem with this definition seems to be that one's utility function not only defines what would be the best course of action, but also defines what would be the second best, and third, etc.

I would say "utility function" takes all possible actions one could take at each moment, and ranks them from 'worst idea' to 'best idea'. A coherent agent would have no disagreement between these rankings from moment to moment, but agents with akrasia, such as humans in the modern environment, have utility functions that cycle back and forth in a contradictory fashion, where at one moment the best action to take is at a later time a bad choice (such as people who find staying up late reading Reddit the most fun option, but then always regret it in the morning when they have to wake up early for work).