Who has final say over an aircraft flight path?
The pilot has 100% responsibility for how, where and when to fly/land. The movies make it seem like pilots are just mindless drones, under the control of ATC. 

In actual fact ATC are there as an aid to the pilot, helping stop big metal tubes from colliding with one another.

This was borne out (to some extent) on US Airways 1549 where ATC were advising the pilot he could aim for his nearest airfield. His skill and experience (and of course having a front-row seat!) told him he would not make it, and he took the action to ditch on the Hudson river - saving all on board. We will never know what the outcome would have been if he had followed ATC advice, but multiple trips in a simulator have told us it might not have ended so well.

Final say is ALWAYS, 100% of the time, with the PIC.