By which I mean in terms of usage, development, or stability. Also along those lines, which English-speaking instances would you recommend?
Interested as Misskey & forks have some feature edges over Mastodon in my opinion, but it’s somewhat harder to find discussion/info about them.
I’ve heard good things about Sharkey, it’s what blahaj uses for their microblog stuff.
There’s also Iceshrimp, though last I heard they were becoming their own thing written in C#.
The others I know about don’t seem to be maintained. Can’t speak to using them, I find the interface far too busy (default Mastodon UI users).
They are aiming for a launch of the re-write towards the end of the year. Let’s hope it goes well.