The most important thing is that it should have a moderation approach and community rules that you feel comfortable with.
Next most important thing is there there's existing defederations that you do or don't want. (Example: Do you care if your instance has been defederated by beehaw, meaning you won't see communities from there? Do you care if your instance federates with exploding heads (fascists) or lemmygrad (tankies)?) It's hard to predict what will happen in the future, of course.
Third most important would be the technical stability of the instance itself.