Things I personally look for in an instance:

• An active admin that has a level head and seems like a good person (preferably multiple admins to prevent Bus factor)
• Instance runs a fairly recent version of Lemmy. If they run a very old version, that could be a sign of problems
• Is defederated with troll or drama filled instances
• Having a unique theme or vibe instead of being a general server is a big plus that makes it stand out, but not strictly necessary