The Lemmy devs operate lemmy.ml, but it’s far from being the most active one (it’s currently the 5th): lemmyverse.net/?order=active_day
Large instances admins? The most active communities are spread across dbzer0, lemmy.world, programming.dev, lemm.ee, feddit.de, etc.: lemmyverse.net/communities?order=active
The main difference of Lemmy compared to Reddit is the ability that communities have to walk away, as I explained in another comment: discuss.online/comment/5393546