I would want to live in WWII Germany so that I could ask Hitler why he was so psycho that he ordered the mass genocide of his own people.
I would also want to travel to the medieval time period around England and Scottland so that I could see if King Arthur and Merlin really existed and because I find the time period fascinating, despite the large death toll from things like the plague and the discriminatory way women were treated.
Except Joan of Arc, because she was amazing and disguised herself as a man so that she could join the army and help France win the war.