I'm on Mastodon.world. It has been excellent, and the admin team even created some stats pages to show uptime. It's pretty stellar, in my humble opinion.
Mastodon
The project: https://github.com/mastodon/mastodon
Mastodon instance affiliated with Lemmy.World: https://mastodon.world/
Discuss the Mastodon platform here. Follow the lemmy.world rules.
Other than a hiccup from a bug the other day where nothing I was posting was actually being posted, I've had no issues with the instance I use at all. It takes a bit to pull new content when I am the first and only person on it to have subscribed to something not already crossfeeding with it tho. But I think that would be the same on any instance as it's just the way the whole federation thing works. ๐คท๐ปโโ๏ธ
If you can't see it, I'm on yiffit.net.
Apparently my instance had some issue over several weeks which has been fixed some time ago. Now everything is in an acceptable range again.