Definitely Zenos resummoning Shinryu, and then flying to Ultima Thule on his own without a guidance crystal nor knowledge of space travel.

Computer guidance system? Nah. Just becomes Shinryu, obtains escape velocity, and travels upon, "the light of the stars."

Our Hunt ends this day. There is nowhere you can flee that we cannot follow!

Put more eloquently:

One man turns into a draconic primal originally summoned as the embodiment of hatred, flies to the furthest point in the universe, bids the Savior of Etheirys to ride upon his back, and chases down a god-like being composed of pure despair.

Of course, perhaps you think of all that as just epic rather than audacious.

Well, how about the time the same guy decided to keep that same primal as a trophy to be unleashed and then used... not for some grand purpose that any other primal was ever invoked for, but for a singular contest against a single warrior.

We tower above the gods! You by your gift, I by my might!

For Zenos, Primals are play things. If that's not audacious, then I don't know what is!