Dolores Umbridge
Lord Voldemort was never the headmaster, Severus Snape was the headmaster when Voldemort was in power. After Snape ditched his position before the battle, McGongall automatically became the headmistress. After the war, she held this position formally for a few years before retiring. It is unknown who was the head after her.
It is unknown. Minerva McGonagall retired and was not headmaster during the epilogue.
Harry Potter was never headmaster of Hogwarts. After defeating Voldemort he became an Auror and then later, Head of the Auror office. It is unknown when he died, although he was still alive in 2017 when he was 37.
The only thing said about Hogwarts teachers is that Neville Longbottom is teaching the Herbology class. Nothing about who the Headmaster is, though. it may be McGonagall or Flitwick but that's just guessing
Dolores Umbridge.
professor snape
Transfiguration was taught by Professor Albus Dumbledore. When he became headmaster he was replaced by Professor Minerva McGonagall.
Dumbledore, because he used to be a teacher (before Harry went to Hogwarts) but then got promoted to Headmaster.
He was Professor of Transfiguration.
harry potter
severus snape takes over as headmaster once harry leaves school to search for the horcruxes.
Severus Snape is the headmaster of Hogwarts in Harry Potter and the Deathly Hallows.