You said one or two so I will give you two. So hard to limit it to just a few very hard to chose.

Madikwe Hills:

Number one: All at the same time - grand central station of game viewing all this visible from our land rover at the same time.
A brand new lion kill. Seven lions including cubs gorging on the fresh kill in front of a watering hole.
Six rhinos with a new rhino baby probably only a few days old that could barely stand.
Two flamingos in the water that were obviously lost.
One jackal howling for help from his friends.
A tree full of vultures waiting their turn.

Number Two: Probably only important to me but seeing a Brown Hyena come to drink and a Brown Hyena pup come out of the den - incredibly rare sightings.