The only important thing, really, is that the instance you choose doesn’t defederate with other instances for political reasons (except being literal Nazi instances). I find that lemm.ee users have a good mix of political expression ranging from Marxists to Moderate Conservatives. An account there, on lemmy.zip, or lemmy.ml is probably your best bet.
Well, one other thing is that instances will only fetch content from communities that at least one instance member is subscribed to. Every instance will have a slightly different selection of content because of what communities the existing members have subscribed to (and larger instances will typically have a broader selection of content, because they have more users).