So I created an account and asked Smogon and this is what they said.
Which UB is the best is largely based on the format you're talking about, since many of the Pokemon banned from OU are unviable in Ubers, while other OU-legal UBs are more useful in Ubers.
It's probably Pheromosa if you're talking about something based on OU though.
Keep in mind these are the tiers for UBs.
Nihilego is in UU
Pheromosa is in Ubers
Buzzwole is in OU
Celesteela is in UU
Xurkitree is in RU
Kartana is in OU
Guzzlord is in PUBL
Naganadel is in Ubers
Stakataka is in NU
Blacephalon is in OU
Thanks SmellyJacket
Naganadel and Pheromosa are both banned from OU, however both of them are pretty useless in Ubers. Based on Pheromosa's Smogon page, it is clear that Pheromosa is only viable because of its speed, allowing it to pivot in and revenge kill opponents with U-Turn. It also has an incredible coverage movepool, allowing it to target many different Pokemon, from Calyrex-S and Lunala to potent Dragons like Zygarde and Eternatus. However, it's frailty allows it to be pretty easily revenge killed back.
Naganadel on the other hand struggles immensely to be of any use. It is the inferior Poison-Dragon type to Eternatus in Ubers. Because of its weak defenses, it is easily countered by threats such as Groudon and Excadrill. Even with a speed boost, it is outsped by Choice Scarf Calyrex-S, who can OHKO it. Naganadel relies on Nasty Plot to be exceptional, which allows it to be KOed while it's setting up.
Clearly, the best UB is Pheromosa, however it is very niche in it's tier, even if it's Ubers.
Please tell me if I have made a mistake in describing competitive Pokemon