Just for you Heather, you can list 5 :)
The delicious primordial soup on the verge of making self-replicators ----but there is a possibility that I risk watching a non-event, if it turns out that life came via comets :)
Well, then in that case, it might be an awesome show to watch (that is, if you aren't so close that you're vaporized and/or obliterated in the course of watching the impactor event!).
Hmmm, maybe I'm just hung up on technicalities, but I always wonder about the non-linearity of such possibilities. Like what if I go back to watch eukaryotic life emerge from bacteria and archea, and I accidentally step in the pool at the time it's about to occur and make it not happen? Or, in the Red Dwarf land of possibility (Ouroboros!), what if I end up creating eukaryotic life if I go back to try to witness its beginning?
So I guess my answer is: the beginning of eukaryotic life.
But a close second option would be to go back and watch the launch of Apollo 11 and subsequent TV broadcasts from the moon...or, if I could get sufficient clearance, to meet the Chief Designer himself... Sergei Korolev.