"Everyone is splendid, aren't they?"
みなさんりっぱですね / みんなりっぱですね sounds most natural to me.
If I wanted to use a particle here, I'd use が: みんながりっぱですね , because は is really a contextual particle and using it would imply a previous context instead of a simple statement, and/or a contrast ("everybody is great, unlike...").
Without context, we don't really know, but が would be much better for a simple matter-of-fact standalone sentence.
みなさん is the more formal version; it's actually sonkeigo (honorific speech) while みんな is the normal version.
^link to honorific speech explanation