I'm on mas.to, which is, I believe, the second largest after mastodon.social. I picked it because it's got the shortest URL, but it has been quietly reliable and up to date. Its moderation and defederation policies are stricter than some might like.
I follow many people who use infosec.exchange and hachyderm.io and they seem to be nerd-oriented instances. But it doesn't matter all that much, since you can follow and interact across the whole federation. I'd say just pick one that's reasonably large and old enough to have proven it is run by people who know how to run a reliable service and are in it for the long haul. It's possible to move to a new instance with only a little inconvenience, so don't agonize too much over it.
I follow many people who use infosec.exchange and hachyderm.io and they seem to be nerd-oriented instances. But it doesn't matter all that much, since you can follow and interact across the whole federation. I'd say just pick one that's reasonably large and old enough to have proven it is run by people who know how to run a reliable service and are in it for the long haul. It's possible to move to a new instance with only a little inconvenience, so don't agonize too much over it.