I'm not particularly fond of any of these, but whatever.
Snow Warning Kyurem: This is excellent flavorwise, but I'm kind of skeptical about using him. On one hand, he has a great special attack to work with, and combined with STAB Blizzard in hail, that's pretty scary. He also has Dragon Pulse, Focus Blast, and Earth Power to hit quite a few things hard, as well as Roost for longevity. On the other hand, he has a terrible defensive typing, cursing him with a Stealth Rock weakness, as well as Fighting, Steel, Fairy, and Dragon weaknesses. His STAB coverage means Steel-types can come in fairly easily and either set up on him or destroy him with their STAB. He'd also be pretty predictable, meaning a special wall can come in and stall to their content. And even with Snow Warning, he faces competition with Kyurem-B, who can threaten both physical and special walls very well, and is threatened by Mega Tyranitar, since if he Mega Evolves, that gets rid of the hail, and gives him an opportunity to set up a Dragon Dance. The best out of this batch, but that's really not saying much.
Nasty Plot Tentacruel: His offensive power was always a bit underrated, but I would never say it was really good. Nasty Plot hopes to remedy this a bit, but it can only go so far. Whenever Tentacruel comes in on something like a Scizor or Mega Mawile, he really just sits there and takes hits after he burns them. With Nasty Plot, he can spend his time powering himself up and hit like a truck, and since he's a Poison-type, he's immune to Toxic, being a nightmare for stall teams. But if he's facing an offensive team, a Nasty Plot set is kind of dead weight. Long story short, the bane of most stall, the punchingbag of most offense.
Gale Wings Salamence: This actually sounds rather interesting. I'd still never be caught dead with Aerial Ace on him, but Roost and Tailwind have potential. Defensive Salamence could use prioritized Roost to heal before most things, and Tailwind can really support the team. But defensive Dragon-types are rare for a reason, since Fairy- and Ice-types are always in the mood to kill a Dragon, and opposing Dragons consider defensive sets easy prey. Stealth Rock weakness does Salamence no favors either, and is still vulnerable to every status in the game with no defense against it. Overall, it's alright, but not great.
Magic Bounce Musharna: It's so hard to not compare her to Cresselia it's not even funny. Moonlight? Check. Same type? Check. Defensive? Check. But there are differentiations between the two. Musharna has Heal Bell, Hypnosis, Yawn, and much more special attack, and with Magic Bounce, it gives her a niche over Cresselia. But now she has competition with Espeon as a Magic Bouncer, who has much more special attack and speed. And despite the buff, I still see her as a poor man's Cresselia, who has much more bulk and speed, and has Lunar Dance. This is kind of like putting Creselia and Espeon together, but getting the minimal results.
My vote goes to Snow Warning Kyurem, because his buff is the one that's the least disappointing.