I agree with Hydreigon for A-. It's one of the few mons that can comfortably deal with Aegislash, and on top of that it can ohko a shit ton of mons like Mega Kan most notably but also over half the current A rank mons. Specs dark pulse is really easy to spam too since dark resists aren't extremely common. I think it's power and respectable bulk + speed is enough to keep it at A-. When you only have 3 mons(or less depending on the point of the match), it's hard to switch around and take way over half from hydreigon's attacks. You don't even have to go choiced either, life orb sets are still great and give you more flexibility with your coverage. And i don't think hydreigon has 4mss really. What else would you even need besides Dragon/Dark/Fire/Ground coverage? That hits literally everything it needs to. Sure you could run flash cannon for fairies but you shouldn't be staying in on fairies anyway.
I don't think Hydreigon fits the B description of "requires a bit more team support to bring out their full potential". Hydreigon is pretty easy to fit on teams if you need a hard hitting mon, especially since it's capable of dealing with a lot of things in the meta. Besides basic teammates that synergize with it, it doesn't need any kind of specific team support to do well.
I don't think Hydreigon fits the B description of "requires a bit more team support to bring out their full potential". Hydreigon is pretty easy to fit on teams if you need a hard hitting mon, especially since it's capable of dealing with a lot of things in the meta. Besides basic teammates that synergize with it, it doesn't need any kind of specific team support to do well.