The metric for making this choice should be on how well the console performed commercially as a result of the software (how much of a role the game played in achieving said numbers). Anything less, like how good you felt it was, how fun it was, whether or not it was better than this or that... is looking in the wrong direction. You judge a launch title by its ability to get units into homes, and nothing else.

Super Mario Bros. is that game.

Why?

Because unlike Wii Sports and Tetris, Nintendo was an unknown brand to most of the world. There was no precedent, no consumer confidence or familiarity to aid in sales. You had a brand new, unknown console coming out of a foreign company right after people had decided to walk away from the entire industry. And yet look what happened. And it's all thanks to the power of the plumber.

When it comes to launch titles, SMB stands alone, imo.

Comparing games from different eras is really hard to do. It's like comparing sports stars that played in different times...

I'll go with Super Mario 64 but most of this list is also a good candidate.

Man, Nintendo really is the king of launch titles. It's amazing how many all-timers arrived at launch on Nintendo platforms. I'd rank them like this:

Breath of the Wild > Super Mario 64 > Super Mario World

All three are in my top 15.

Yeah Nintendo really understands (most of the time) that you need a killer app from day 1 to get your console off to a strong start. 

Had Switch not launched with BOTW, or Wii not launched with Wii Sports, they may not have sold nearly as well.

Tetris and Wii Sports.

Why those two? Because many just bought the system for those launch titles - and not much else!

Other launch titles were also great, but they were more like a title out of many, not generation-defining as Tetris and Wii Sports were.

While the correct answer is BOTW, I'm still going with Mario 64. Nothing beat the feeling of running around princess castle in 3D for the first time.

What a horrible timeline that would be. -_-