Someone called it a tech demo in this thread, but I feel that's a bit unfair. Tech demos don't get sequels. And that was a pretty full fledged game. From the "best objective game" perspective, it definitely doesn't win, not that it isn't good, just that it isn't Mario-Zelda level. But from the perspective of which games defined the console they were on and propelled it to the level of success it had, Wii Sports is the unquestionable winner. More so than Mario 64 for the 64, more so than Halo for the original Xbox, more so than Super Mario World for the SNES, Wii Sports' success was practically inseparable from the Wii's, and not just because it was bundled.
I mean the Title of this thread asks us to choose the best "Launch Title" I take that as not asking which is the defacto better game but the game that was best to help launch a system and get it into consumer hands. Sheer quality of game isn't as important as to how convincing was the title to it's intended user base.
That winner is just Wii Sports in my book. I'm not a huge Wii fan....but I played the heck out of Wii Sports. Halo or Mario would be a distant but extremely strong second.
And agreed, I don't know what to say to someone who doesn't think Wii Sports is a full game. It is as about as much full game as any mini game collection title is. Mario Party, Hasbro collections, Sports Champions, etc. And "Demo" be damned, it was the most consumer convincing Demo that ever was.
I'd argue that BotW does compete here though. It's the best selling Zelda game by a country mile, tracking to outsell every version of OoT combined with just the Switch version. At launch, it had OVER a 100% attach rate, due to people preordering the game despite not having a Switch yet. That's a pretty good fucking launch game, I'd say, just in terms of doing the job of launching a console. Doesn't hurt that it's also an excellent game that represented the start of a new era for Nintendo. I'd place it as the solid second on that logic, actually, with Mario Bros., 64, and World all gunning for a close third alongside Halo.
A good launch game literally launches the console into sales success, and also does more than most games in its library to shape the console's identity. Really good ones define eras, either for the company or the industry as a whole, and can win cultural victories as well as sales ones. Wii Sports excels in every category here. Functionally launching the console, defining the console, defining the trends of the 7th generation for the industry and for Nintendo as a company, achieving unprecedented sales success for a single release of a single game, propelling the Wii to victory, and winning Nintendo a cultural victory in the form of extremely mainstream, extremely widespread coverage by news and popular media. Its cultural penetration was unprecedented since Super Mario Bros. practically singlehandedly revived the industry. It's not even remotely close to my favorite game, not even in my top 100, probably not in my top 500, but I do admit it's a lot of fun, and it's extremely difficult to argue with its results.