I’ve been happy with mastodon.world and Lemmy.world has the same admin(s).
I think there are two main schools of thought when choosing an instance:
Smaller more interest based. The advantage here is your local timeline/feed will closer match what you’re into.
Larger general interest. Your feed will be less tailored but you have the advantage of more stability, reliability, and commitment to maintaining the instance.
It’s the typical small fish in a big sea or a big fish in a small sea dichotomy
I read that it's not a good idea to register to the biggest instances so that the user base is spread out. So, I found a nice instance about science and here I am on mander.xyz 🙃
Hardware specs used to run the instance (for supporting active users, the beefier the better and the more reliable it makes the instance from going down)
Instance sign up policies (I prefer closed, requested account creation, because the hardware then comfortably supports the active users, and also mitigates bot account creation spams)
General support for popular federated content and level of defederation (can't use lemmy properly without general federation)
maybe ideology of the instance admin (?) but not really
I joined Sopuli 9 months ago, and they do a really good job with all of my categories.
I also created accounts on the main .ml and shit just works instance but one or the other factors failed there, where it didn't here, so I'm waiting for Lemmy to allow account migration to fully get rid of those.
I'm still trying to figure it out. I joined feddit.uk just because I'm in the UK. Then I realised most of the communities were about politics and football teams. I read about Beehaw and like the sound of that so joined up there. Then realised that they had defederated from lemmy.world (which I understand and am not complaining about). So I created an account on lemmy.world as well.
I guess the thing to figure out is to find the communities you are interested in, subscribe to them and make sure your instance is federated with whatever instance those communities are part of. Then it doesn't really matter which one you join? If you just want to scroll mindlessly through posts from all of Lemmy, I guess you can just find an instance that is federated with everything and set your filter to 'All' and go nuts.
I considered lemmy.ml at first, but it had a message asking people to sign up on a different instance because they were overloaded. I chose lemmy.world because the admins had experience running a decently sized Mastodon server.
The first community I missed on reddit after they stayed dark was ich_iel, and they moved to feddit.de.
To be honest, I kinda regret that choice, and I usually browse kbin.social. Smaller instances fail to provide the varied experience that made reddit great for me.
I did also start out on feddit.de but soon switched over to Lemmy.world due to too many limits when it comes to federation (e.g. general ban on nsfw instances).
Since I can still participate in all the communities on feddit.de this does work great for me
You find a post on <foreign instance> (because the feed on your small home instance is very limited) and just want to upvote it.
You get to the login screen and remember that this isn’t your home instance.
You open your home instance, navigate to the search form.
You try to find the link for the original post that you can enter in hour home instance’s search bar (which isn’t at all easy to find if you don’t know where to look).
You paste the link in your home instance’s search form.
You open the post on your home instance.
You vote.
This is frankly unacceptably complicated for a regular, casual user to just cast a vote on a post.
I had first found lemmy.ml mentioned on Reddit, so I checked it out and found they were overloaded and it was having issues staying up and being responsive. I saw someone mention lemmy.world, so I went and signed up there.
Started off on kbin a few weeks ago but as good as it is, it's in its infancy & currently without an api so as im impatient I checked out Lemmy. Chose an instance without content restrictions, blocking the least number of other instances & being blocked itself by the least number of other instances. No idea if that was the best choice but that's how the dice landed.
I didn't pick Lemmygrad.ml because of tankies. I didn't pick Lemmy.ml because it was the biggest server in April last year, and I wanted to avoid the biggest servers (same thing with other fediverse pages). Other servers had dozens of users and almost no content. The server I'm on had like 500 users and a moderate amount of content. And it was located in Finland, Europe (I'm from Spain).
I had zero clue what instance to start with, and of course within minutes every instance started accusing every other instance of everything which definitely did nothing to help.
Beehaw seemed as good as any and mostly cause it just had a different name than Lemmy lol. After a couple days someone linked kbin.social and I made an account there and found I liked the UI more, plus it has the odd feature or three lemmy doesn't have (or didn't, I don't know how that corner of the fediverse has been developing). Day or two later I stopped using Beehaw and just kbin.
I guess I'll also add in Mastodon for shits & grins: Mstdn.ca. Cause I'm Canadian...that one was simpler lol
Someone invited me to kbin.social, and did the work of creating a community for us. Like 99.99% of the Redditors in my community did not follow, but I did, and more will come as Reddit gets worse and the Fediverse gets better.
Don't overlook the effect of providing an actual helping hand to someone to lessen the transition pains, for people who are legit interested and would feel more welcomed that way than having to face that barrier as their very first task.