Fediverse

I would still go with one that isn't one of the biggest. My general advice is to find one that fits the vibe you're going for, communities you're interested in (e.g. some are focused on art or cybersecurity, etc), or is somehow tied to your locality. It shouldn't matter that much, though some servers will be a little more (or less) strict with things like federation, content warnings, alt text, etc. Usually the server will have some info telling you some of this, and their admin should be linked and likely has a post or two pinned to their profile explaining some of this as well.

I am partial to kind.social, though have opted to run my own instead of joining up anywhere.