I think most of the common ones that get brought up are in the conversation already: Pong, Tetris, Super Mario Bros., Zelda 1
I think if we rip away nostalgia, and move away from looking at games that pioneered only one genre it boils down to this.
Argument 1: Tennis for Two, by virtue of being the first videogame invented, though it wasn't patented and didn't have commercial success.
Argument 2: Pong because it was the first commercially successful video game ever and launched video games as a viable source of entertainment.
Argument 3: Doom because it's what popularized (not invented) the FPS genre, which I know conflicts my earlier statement of not considering games that are genre specific, but shooters are modernly the most played genre of gaming which makes it relevant. This of course could shift in the future and the answer would change to whatever ushered in the commercial popularity of the genre that ends up taking a large grip.
Argument 4: Space Invaders because it's currently the largest grossing videogame of all time with inflation adjusted.