What is the best-selling video game of all time?

This is an answer that may change with time.

The best-selling video game at the time of writing is "Minecraft" with sales of 300 million units.

Top 10 Best-Selling Video Games
(Millions of sales units)

1. Minecraft (300)
2. Grand Theft Auto V (190)
3. Tetris (EA) (100)
4. Wii Sports (82.9)
5. PUBG: Battlegrounds (75)
6. Mario Kart 8 / Deluxe (65.47)
7. Super Mario Bros. (58)
8. Red Dead Redemption 2 (57)
9. Overwatch (50)
10. The Witcher 3: Wild Hunt (50)

https://wiki2.org/en/List_of_best-selling_video_games

Define "video game." It's nowhere near 300 million, but a few handheld consoles sold over 100 million units.

https://en.wikipedia.org/wiki/Handheld_game_console#Timeline