I've made a list of popular communities about 2 weeks ago you might want to check out. Otherwise you can go to communities --> All and start browsing all the popular communities out there.
Note that the subscription count is only from your instance, so some communities are more popular than they look.
There's a site called rehab.sub that allows you to transport your reddit subscriptions and finds equivalent communities across the fediverse and Discord. It includes Lemmy, KBin, and Scribbles for the fediverse communities.
It's the best way to make sure you're not making duplicates. Also try https://wefwef.app/ they have a useful feature in settings to find your old subreddits