Interesting question. So, if you ask for the beginning, the first game, even that is difficult to answer. There is an hour long video exploring this question, and this hour is not boring:
https://www.youtube.com/watch?v=uHQ4WCU1WQc
Pong is a pretty decent answer, as this is a very early game that is:
- fairly successful
- commercial (most of the games before there shared freely)
- had cultural impact