Id say blacephalon is B- to B. It does hit hard, but in terms of power it is generally outclassed by aegislash/heatran. It does have a niche in some rain and chalkf variants, and can steamroll with proper support.
Nganadel is B+ to A-. Nowadays often replacing amoongus on chalkf teams, it offers support and setup with nasty plot. If supported properly (often just a foing partner) and can decimate the opposing team. Its lackluster bulk prevents it from being as common, as it that generally interfers with the general bulky synergy of chalk teams.
Stakataka is B to B+. It has a niche on sun teams, and if properly set up can deal massive damage. The issue holding it back is that nearly every mon has a coverage move that can ohko it, and wigh lando-t being so common it often steuggles keeping its damage up. Wg buff also doesnt help.
Kommo-o even with its z move is incredibly underwhelming, still gets ohkoed by fini at +1. The prevelance of tapu fini, aegislash, tapu lele, metagross, salamence, and even kangaskhan really dont help it at all. I would rank it as B- or C+ at best.
This opinion comes from extensive laddering in the high vgc18 sd ladder.