Perhaps the last 3 days in Hitler's bunker.

Or maybe better, to be a fly on the wall at the Yalta Conference.

Or Himmler's bizarre speech of 6 October 1943 at Posen, where he called out to Albert Speer, so I could see with my own eyes if Speer was lying when he later claimed he wasn't actually there and that Himmler's eyesight was so poor that he didn't realize Speer was missing. Speer laughably relied on this claim to demonstrate he knew nothing about the Holocaust.

I know that last one sounds like I'm thinking small, but it is an issue Adam Tooze raises in his book Wages of Destruction and that book really piqued my curiosity to know.

Another might be the death of Stalin's wife, so I could settle once and for all whether she committed suicide or met with foul play!