beehaw.org is a lemmy instance explicitly chartered as encouraging politeness and diversity. They are also tightly curating the locally hosted communities -- none of these thousands of copies for subreddits, just a couple of pages of very carefully chosen communities. But, for all that, traffic seems pretty high.

kbin.social (and other kbins) are not Lemmy, they are a competing code base that uses ActivityPub in a way that is mostly compatible with Lemmy and can federate with other Lemmy instances. kbin has no externally facing API, so there are no third party apps that will work with it (and so far, no prospects for them at all). It's pretty similar to lemmy.world, but maybe quieter, and with a more technical user base. Arguably the web site works better, as the code is a bit more stable.

lemmy.ml is most similar to lemmy.world, it also seems to have a more technical user base. Has a lot of Linux and FOSS-related communities on it.

lemmy.world you know about :-)

UItimately, you can get to most communities through your lemmy.world account, as lemmy.world is federated with the others. But I do find myself logging in directly sometimes to get the best experience.