Ok so instead of my normal huge posts, I'm just gonna post about one: Cress.
Cresselia is a really underrated Pokemon now. Ever since the Aegislash ban, Psychic-types have skyrocked in viability, and this is for two reasons. The first reason is the more obvious one; there's one less Pokemon that can abuse Cress. Aegislash resisted Psychic and Ice, is immune to Toxic, didn't really care about Thunder Wave or Hidden Power Fire, and could kill it very easily with Shadow Ball. Now, Cress doesn't have to worry about Aegislash any more, and the other two viable Ghosts don't particularly enjoy dealing with Cress. The second reason is that Aegislash's removal also made Mega Medicham much more common, and so having a Pokemon that resists High Jump Kick and Zen Headbutt is a necessity on every stall team. Many people choose Slowbro for this role, since it also acts as a bulky water, some choose Victini, due to the extreme offensive presence, access to Will-o-Wisp, and Fairy resistance, and some choose Mew, due to its ability to terrorize opposing stall teams. However, Cresselia is also an incredibly viable choice for the role. Cresselia can counter Mega Medicham extremely well, while also countering Charizard-Y (it also can abuse Moonlight's recovery with the sun's boost), Mega Gardevoir, Landorus, Talonflame, Latias, Landorus-T, Latios, Greninja, Garchomp, and some other offensive threats. Of course, it's a defensive Psychic-type, so everyone always talks about how it's complete Pursuit bait. However, its defensive set commonly runs Reflect, which allows it to flee from Pursuit users in relatively good health. Being a defensive Psychic also makes people go crazy over the Knock Off weakness. However, it doesn't mind the move that much, since STAB users of it are somewhat rare now and coverage variants are relatively easily brushed off. An extremely good quality of Cresselia on stall teams is that it hard walls Calm Mind Landorus, which no other stallmon can do. Calm Mind Landorus isn't extremely common, but if you're running a stall team without Cresselia, you will lose to it. Cress also has a couple other pros, like being able to run a good dual screens set, access to Lunar Dance which is great for HO, and Trick Room, which is niche as hell but decent.
I'm not going to just say its pros though. Cresselia obviously has its flaws that prevent it from being a top-tier threat. The most major of these flaws is its incredibly unreliable recovery. Cresselia has the bulk to stand up to the likes of Excadrill, Mega Garchomp, and Talonflame, but sand restricting its recovery is pretty terrible. Moonlight also has low PP, and furthermore is restricted by rain. The only benefits of it are the ability to abuse Charizard-Y's sun, but that isn't that important, considering you avoid the 2hko anyway. Cresselia's offensive presence is also pretty terrible; it can't 2hko stuff like Amoonguss, can't OHKO dragonite after rocks, and only barely 2hkos Keldeo. I've also been stressing how Cress likes its Psychic-typing for Medicham, and it does. However, outside of Cham and Landorus, which again are huge, it relies on tanking super effective and neutral hits a lot. It isn't weak to that many attacks in OU, but it also only resists/is immune to three types, two of which are relatively rare.
When you look at Cress and try to rank her, you see her pros and cons, and have to see how they balance out. On the negative side, it's hurt by unreliable recovery, slight 4MSS, and a very specialized defensive typing that leaves it neutral to most moves. On the positive side, it's able to fulfill multiple niches on stall teams: it's the only stop to CM Landorus, and it also counters multiple other Pokemon, such as Mega Medicham, Charizard-Y, Mega Gardevoir, Landorus, Talonflame, Latias, Latios, Landorus-T, Greninja, and Garchomp, and can check, less reliably, pretty much every other offensive Pokemon that doesn't use Dark/Ghost/Bug STAB. It does struggle against stall teams, but that isn't as important as its handling of offensive teams, since its job is to be a glue mon on stall teams able to counter, or at least check, almost everything in OU. I think its pros outweight its cons, and so, in tandem with its need for some support (something to handle Bug-, Dark-, and Ghost-types as well as weather teams), I think it fits in the B rank perfectly. Comparing it to mons like Staraptor and Celebi is somewhat hard, so I don't think it's a great fit for B. However, I think its capabilities on stall teams equate that of Chesnaught; in fact, they work pretty damn well together. For the reasons above, I think Cresselia should move up to B-.
