I voted for Pong, but, the thing with these games is, that I think people will choose them for different reasons.
For instance, as notselfcreated said, Pong has become a pop-culture reference to the gameplay of outdated games.
Pac Man, of course, is a symbol of the arcade boom of early video games.
Mario, I think, is less a historic waypoint in video game history, as it is a very recognizable video game character. From the clothes, to his signature sound-effects, everything associated with Mario, is instantly recognizable, without ever mentioning or showing Mario himself.
Space Invaders, I think, is a somewhat-underappreciated entry. It didn't symbolize the arcade revolution like Pac Man did, and, although it was a hugely popular game in its time, I don't think it reaches into modern pop-culture as far as Pac Man does.
Frogger seems like a rather random candidate. Sure, it's stood the test of time, but I could never call it more recognizable than some of the earlier mentions.