For better or for worse (obligatory remark on how in a federated model there should be no “main” or “flagship” instance) that would probably be lemmy.world on the sheer number of users alone?
The creators of lemmy run .ml. Its very existence is the basis of all others. From what I can tell, ml is also self hosted. Most likely by one of the devs. So like, right now, this conversation is getting passed around on his computer in a closet somewhere. Think about that. Yeah, .ml will always be the main instance regardless of how anyone thinks about it.
Memes matter. You wouldn't think so, but they actually do.
Lemmy.world is the biggest and most active instance by far. But calling it a default is not accurate.
If a default emerges, that's a problem for the platform. Federation works best when there are many valid, major instances with slightly different flavors.
But I will admit that lemmy.world is carrying the activity of the platform, without their userbase we would be in trouble.