If I'm able to 'witness' without consequence of being in the timeline (and have the magical universal language translation device) there are few I'd like to attend for various reasons (awe and wonder, truth of events, or simply experience it). I'll skip most other that have been mentioned.
Gobeki Tepli as the Comet fragments were approaching
https://www.telegraph.co.uk/science/...0950bc-wiping/
Construction completion of the Sphinx and the Giza Pyramids - just how did the culture / societies in question celebrate their feat?
Hammurabi in 1790-1752 BC understanding the contextual base from which derived the code of laws
Solomon in 970 BCE - seeing what his 'schedule' was like with 300 wives and 700 concubines!
Jerusalem during time of Christ and subsequent Roman conquest and destruction of temple in wars of 67-70 AD see if Josephus account is accurate.
Ghenghis Khan - Be in a village on the plains of Asia to experience the sheer terror of the Mongel Hordes descend en mass
Leonardo Da Vinci - see him working his craft
Lincoln Gettysburg address - how many actually could hear it in person with no sound system?
Observe Nicola Tesla at work in lab at Colorado Springs and Wardenclyffe (as well as other inventors in their moments of inspiration)
See who and why James Forrestal was murdered in 1949
Was James Vincent Forrestall Murdered For Wanting to Release UFO Information? Video | <b><i><a href="http://www.educatinghumanity.com">Educating Humanity</a></i></b>
On the Navy ship July 17, 1996, whose information is classified
National Security Invoked Re Navy Activity Near Flight 800