I don't know of every instance of course, but from the general purpose ones the order of up-to-date from most to least at the time of posting would be something like: kbin.run, fedia.io, kbin.cafe, artemis.camp (specific API branch), kbin.social.
But there's also other considerations. fedia has a double post/comment issue and a low amount of 500s. Artemis has been giving me a lot of 500s, can't upvote anymore (just reloads the page), commenting doesn't always federate. Many instance admins seem to have disappeared since 3 months ago. I'm not sure what life is like on instances I'm not actively using so there might be issues unknown to me.
Edit: the keys that I look for are localization / ui changes. "more" dropdown now has "more from domain" on updated instances. The UI for the sidebar is different, and most recently the footer is differently styled. There's also the federation list of instances page in the footer now.
VulcanSphere