That's a loving bold question. I don't think there is A best video game of all time, rather several.
I'd say:
The Portal franchise, due to its overwhelming storyline, amazing level design, and characters.
Half Life 2, due to it still being an incredible game to this day. I'd say it's one of the games that shaped PC gaming because of how groundbreaking it was and in some ways still is.
Fallout: New Vegas. I think that one explains itself. The overwhelming amount of playthroughs you can do and choices you can make, the overwhelming amount of dialogue involved, the unique world you are thrown into and the many DLCs which add to the many hours you can clock in all just blend together to create a really loving good game.
Team Fortress 2 as it inspired hundreds if not thousands of team-based FPS games of its kind, still sports a booming playerbase, and has thousands of community servers and community-made addons. I know the meet your match update seems like its death, but the community servers and the MANY community and Valve-made maps you can play on and have so much fun in really form a fun experience still.
