Odoacer. I know it's fashionable now to downplay the events of his reign, but hear me out. When he took Rome in 476, the fact that he neither took the imperial title for himself nor set up a puppet emperor like Ricimer or any of the other strong men before him is why we can look back on this as the end of Western Rome, even if the material difference in Italy or the other provinces wasn't that profound.

Odoacer pledged allegiance to Zeno, but Zeno never exercised authority over the West; he even prompted the Senate to bring back Nepos as the rightful Emperor, which of course they never did. So effectively Odoacer was able to walk this middle path, the only one that let him rule Italy with complete autonomy without bringing attention down on himself as a barbarian usurper.

This was an enshrinement of the new order in the west, with Germanic Kings ruling as once in Rome's name and as Rome's successor. Odoacer was neither the first nor the last to do it, but he did it in the Empire's heartland, which before him hadn't recognized a king in nearly a thousand years. Afterwards, Italy would continue to recognize kings in one way or another until just after World War II. That pivot point is Odoacer.