1500 (an arbitrary round number)
1492 (first transatlantic voyage of Columbus; end of Reconquista)
1453 (fall of Constantinople)
1517 (Protestant Reformation)
1485 (Battle of Bosworth Field)
1516 (death of Ferdinand II)
1504 (death of Isabella I)
Except for 1453 all these ending dates are West Eurocentric, as is the focus on Western Europe in the definition of the Middle Ages. That geographc focus is too restrictive. Western civilization started in northeast Africa and southwest Asia (see my chronology here). Any account of Western civilization should cover everything that happened between the Iranian Plateau and the Atlantic Ocean.
IMHO if the Middle Ages are supposed to be what happened in the West between the effective end of Roman Empire hegemony over the Mediterranean Basin and the rise of modern Western civilization, they should be defined in terms of as when Christianity and Islam fought for supremacy between the Iranian Plateau and the Atlantic Ocean. In any case, there was no abrupt transition from antiquity to medieval or from medieval to modern. The transition periods should be defined by ranges of dates rather than single dates.
So here are my proposed ranges, both of which are conveniently 297 years long.
Transition from antiquity to medieval:
325: First Council of Nicaea: Christianity is standardized sufficiently for it to become (towards the end of the century) an official state religion
622: Hegira
At the end of the first transition period, the two transnational Abrahamic religions are in place and ready to start fighting over what used to be the Roman Empire.
Transition from medieval to modern:
1492: First transatlantic voyage of Columbus (start of European takeover of the rest of the globe); Fall of Grenada (end of Moslem rule in Western Europe)
1789: French Revolution (The Europeans now have finished assembling the social and political institutions that will enable them to finish their takeover of the rest of the globe. In particular, what used to be Christendom, but now is partially secularized, now is overwhelmingly more powerful than any other society generally and the Islamic Ummah in particular.)
If you still insist on a single date, 476 is as good a date as any for the start of the Middle Ages. The usual dates for the end of the Middle Ages are about 150 years too soon. My candidate is 1648 and the Peace of Westphalia, in which the countries of Europe agreed to stop fighting over religion, thereby formally abandoning the legal fiction of a single renewed (Holy) Roman Empire with a single (Catholic) state religion.