If we were in a different meta, where Hail was banned, I could see Cresselia being a C-Rank. However, with Hail being re-introduced, and Abomasnow running around on ladder making it more popular than it was before, I can't see Cress going anything above D-Rank. It does have awesome 120/120/130 bulk, but it has absolutely 0 offensive presence without getting a lot of boosts in. Chansey didn't have much of an offensive presence, but it had nearly unlimited support options in 352 HP Wishes, Thunder Wave, Toxic, Softboiled, heck, even Counter for a gimmicky way to get rid of certain physical threats. Cresselia does not support her team to the same level Chansey does. it has Thunder Wave and Toxic. You could use Dual Screens, but most of the Pokemon that appreciate having screens are frail, offensive teams, which Cresselia does not fit well on. You have Lunar Dance as well, but are forced to KO yourself when using it.
Furthermore, of the Pokemon listed as S tier and A Tier, Cresselia can't wall very many of them. Heracross has Megahorn (saying it can wall Heracross locked into CC shouldn't raise its tier. Anything that resists Fighting can do that.), Crobat can use Taunt, SubCM Raikou will use Cress as setup bait. Down in A-Tier, Chandelure, Cofagrigus, Sharpedo, and Weavile have distinct Type advantages over Cresselia. On top of this though, Togekiss can set up and Flinch you to death, Venomoth can either put you to sleep, or set up and threaten to Bug Buzz (or just pass it off to somebody else.), and you can't reliably beat Shaymin between SpDef drops and SubSeed. A handful more of these Pokemon are "walled" but they'll just U-turn or Volt Switch out.
I think the main thing that puts Cresselia at D-Rank though is its typing. In the Mew discussion thread, you heard people talk about how Psychic is decent typing. Offensively, this is true. It hits a majority of UU for neutral damage, and you can use coverage options to hit what you can't. Defensively, though, Psychic is really appalling typing, since you sport weaknesses to many common attacking types in Bug, Dark, and Ghost. I can't see Cresselia going above D-tier because it's supposed to be a defensive Behemoth, but its typing does not support this in any way.
I don't have enough experience with Amoonguss to make any real comments, but between Spore, Clear Smog, and Regenerator, I could certainly understand a B-rank, but I think C-Rank would be more appropriate, since again, it has 0 offensive presence and while Regenerator is good, 114/70/80 defenses are not that great.
[18:01:20] <NDEBeach> i remember
[18:01:27] <NDEBeach> redshark used to be the good device
[18:01:30] <NDEBeach> AR was meh
[18:01:35] <NDEBeach> wait