It probably would have been beehaw if they didnt defederate so early. Given the viewpoints of the people in charge there expressed in their comments on defederating and refederating, I suspect that Beehaw is going to have a consistent problem with constantly defederating and refederating.
Lemmy.world is most likely to grow the largest now because the barrier to entry is low compared to the other two. Additionally, the instance name gives the impression of a general or catchall instance moreso than lemmy.ml or beehaw.