Kyurem-W is a wallbreaker among many, and far from the best in any sense. It's very powerful, but its most powerful move leaves it too weak to KO much anything. It has rather awful defensive typing, and a SR weakness is always a limitation (Ho-Oh is much, much better than Kyurem-W, let's not even go there). For all its power it won't break Water Arceus, RestTalk Kyogre, Chansey or Blissey. Ho-Oh can switch into any of its moves and pivot out to an appropriate resist (essentially anything if you use Draco Meteor, a Water/Steel type if Ice Beam, or it can stay in on any other move), taking little damage overall. Fairy and Rock Arceus trouble it a lot. With Fairy- and Steel-types as well as Fire-resists almost ubiquitous, it's forced to predict a lot. In addition to all this, it needs to find a justification to be used over Kyogre, Ho-Oh and Zekrom as a wallbreaker.B-rank is way too low for Kyurem-W.
I mean Zekrom and Landorus-T are A but Kyu-W B? That's just crazy.
Is it because of the SR weakness? Reliable Defog support is disgustingly easy to get in ubers, otherwise we wouldn't have Ho-oh in A+ (and it was considered for S before if I'm not mistaken).
Xerneas? Takes massive damage from Ice Beam and even if it does switch into Draco Meteor then it becomes Ho-oh/Aegislash bait, who should always be paired with Kyurem-W since they have amazing synergy together. Being paired with one or two of the best pokemon in the tier certainly doesn't count as a flaw.
95 base speed is just good enough to outpace the oh-so-common base 90's (let's be honest, every uber team is going to carry at the very least one of those) and it can be either a powerful wallbreaker with Specs or a solid revenge killer/lategame cleaner with Scarf. If you think it's one set and it ends up being the other you're in serious trouble.
Actually, lots of stuff from B-rank needs to be moved up. It's absurd that things like Kyurem-W, Rayquaza, Heatran and Lugia are in the same rank as Whimsicott of all things.
Anyway I strongly believe Kyurem-W should be moved up to A-rank.
About the Arceus formes, I can say for a matter of fact that Arceus-Ice is more viable than Psychic since it's really not outclassed by anything else unlike it (Kyurem-W is a hit-and-runner, Arceus-Ice is a CM sweeper). For example why on earth would I run Arceus-Psychic over CM Mewtwo? What else does Arceus-Psychic do that another Psychic type doesn't do better? That alone means Psychic should be ranked below Ice.
Either way I think it's a bit too early to rank the more obscure Arceus formes since they may or may not have a niche that makes them worth using over the standard ones like Normal and Ghost.
Rayquaza's premier set of the past generation (Mixed Wallbreaker) is checked effectively by Fairy Arceus, Rock Arceus and Water Arceus, one of which is present on most balanced/stall teams. Its Dragon Dance set is very difficult to set up, will still lose to certain Choice Scarf users (Xerneas and Genesect, mainly) and depends on Outrage for a lot of OHKOs, which means that even if it manages to set up it can never sweep while the opponent has a Fairy-type. Its Swords Dance set is just as difficult to set up, is heavily dependent on a somewhat weak Extremespeed, is still checked by all the aforementioned Arceus formes and is usually inferior to Arceus-Normal (Dragon Claw and V-Create are very powerful, but Arceus-Normal is far easier to set up and has a more powerful Extremespeed).
Heatran is something of an anomaly in that its typing and bulk allow it to effectively check some sets of major threats, but not all. Xerneas with Fighting-type coverage and Ho-Oh with Earthquake destroy it, meaning a secondary check is needed for both, while Ho-Oh checks many of the things that Heatran does check reliably just as well. As cool as its typing and movepool are, this makes it difficult to fit.
I addressed Lugia in my earlier post.
Arceus-Ice is outclassed by Arceus-Electric, which has the same coverage with a typing that at least has a few somewhat useful resistances and only one weakness, as well as not being weak to Stealth Rock.