I’m on calckey.social … now probably firefish.social (as in they’re making the change today).
It’s the main flagship-style instance. But they’ve been having performance issues for a month or so that aren’t fatal but not ideal. There’s a good chance the move to the new server (ie firefish.social) will fix all that but I wouldn’t guarantee that.
Other than that I’m not really sure. blahaz.zone seems to be run well. the admins of infosecexchange and mastodon.world also run instances, but I’m not sure how they’re going. So, firefish.social is probably the best recommendation … though there are actually a number of instances out there!