I believe it is going to be Lemmy.World. Besides my home instance of Kbin, which I love dearly, it is the one I see and hear about the most.
It does make sense because it is open to sign-ups, is federated with pretty much everything (apart from closed off instances like Beehaw and the tankies at Grad), and has the advantage of being a Lemmy instance (which gives it name recognition).
In the long-term, I hope federation keeps this space open, even if there is one or some "main" instance(s) people use.
I read this in a Harley Quinn accent so I'm 99% sure this is the actual Margot Robbie and is all the proof I need to consider this a verified account. Thank you Ms. Robbie for explaining things once again like you did in The Big Short!