Lemmy seems like the front-runner, but wait. Wait wait wait.
lemmy.world is perhaps the new instance where people are joining the fastest. But Lemmy.world was just defederated from Beehaw.org. I think this event is key and serves as a necessary discussion point for anyone jumping into the Fediverse.
The rules are different. Beehaw.org has some of the most popular groups on all of lemmy right now (including !programming@beehaw.org). By defederating from lemmy.world, it means that anyone registering at lemmy.world can't access !programming@beehaw.org.
Beehaw.org seems to me like tildes or Lobste.rs, where they're trying to create a carefully curated group of users. So the open-registration model at Lemmy.world is not currently compatible with Beehaw.org.
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Lemmy.one however, is a more curated list of users. But unlike Lemmy.world, Lemmy.one (and Beehaw.org for that matter) cannot create new communities. Lemmy.one also has a simple interview process (you have to answer the question "Why do you want to join Lemmy.one" before you're allowed to join their group).
My recommendation: try to join a "trusted" server, like Lemmy.one, that has broad access to different Lemmy-instances. Lemmy.one may not match your philosophy, but find one that does.
Lemmy.world, and its "open registration" policy is likely best for any Reddit refugee who wants to play with Lemmy. Since registrations (and community-creation) is fully open, you can get started in less than 5 minutes (No interviews, no worried about online philosophies / political slants, etc. etc. Just create an account and go). But this laisse-faire has led to spam-bots being created on Lemmy.world, and those spam-bots coordinated a porn-invasion of some Beehaw.org groups. So Beehaw.org doesn't want to deal with users from that open instance. As is their right in the Fediverse.
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So yeah. Its not quite as easy as people are making it sound. But its really not that complicated either.
1. Join a "trusted server" if you're into that. Lemmy.one could be a good starting point. I see others here are recommending programming.dev (also a Lemmy instance, and likely with "good behavior" rules to stay in Beehaw.org's good graces).
2. Join a "freeform server" if you're into that. Lemmy.world is one. But note that you'll be less trusted and less likely to access !programming@beehaw.org. Actually, there's no reason you can't joing #1 and #2 at the same time, because freeform servers like Lemmy.world have a much easier time on "creating new subreddits" (aka, easier to make new communities to experiment with).
3. Spin up your own instance. It seems like beehaw.org is still allowing most hobby-instances federate with them. This seems to be one of the better solutions, as your own instance can get all the large curated communities from beehaw.org, but also the freeform communities at Lemmy.world or sh.itjust.works
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Sorry for exposing "Nerd Drama". But I think its important to think about how Lemmy and its community plans to deal with this problem. There will be more trolls (and coordinated trolls) from different servers attacking each other. I mean, I used to play video games where we launched DDOS attacks at enemy guilds. Online trolls are just assholes like that, and the question of how to build tactics to counteract hate-brigades is interesting.
Even if I don't agree with anything Beehaw.org is doing, I still support them from the perspective of "I wanna see if they can figure out a solution to this online-hate / spam / trollfest problem" using Fediverse tools. And I think they have a shot at solving it.
So this makes Lemmy _very_ different from Reddit. I think the ol' Reddit philosophy was to grow thicker skin and try to ignore the trolls, recruit more moderators to stand guard and click the ban user button 1000 times as 1000+ spam users bombarded you. But I'm glad that Fediverse offers a new tool (ie: Defederalize) that is between a "set instance to private" and "keep things public" mode.
But its clear that the community is still learning how to use these tools, how to organize our community with those tools in mind, and what tools need to be built so that everyone's happy.
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kbin.social is another Fediverse that's somewhere between Reddit and Twitter (or Lemmy and Mastodon). Due to case-sensitivity on kbin.social, Lemmy bugs out on CaPiTaLiZeD@kbin.social groups, but both are Fediverse and there's a degree of cross-comment activity.
Ultimately though, kbin is likely to remain segregated from Lemmy due to these incompatibilities, and I don't see them fixing it unless they start from scratch and somehow get lemmy+kbin to agree on names (lowercase only vs case sensitive). I'd say give kbin.social a try as well, there's something to be said about trying to be compatible with Mastodon and maybe using the larger Mastodon instances to seed that community.
There's curated communities like Tildes, Lobste.rs, and of course Hacker News. But I don't see these as Reddit replacements.
Beehaw.org fits itself in a weird spot. Its Defederating, so its kinda-sorta like Tildes or Lobste.rs, setting itself as private and making it harder to join to help curate the users. But its still theoretically possible to federate to them (ie: through Lemmy.one, but not through Lemmy.world). I'm going to keep a close eye on this drama to see how the community resolves it.