I have been using Cresselia in multiple teams with multiple sets, and have to say, its really good, so i am nominating it into A+. I explain under, why.
Cresselia is very good, while she is not dominating the VGC as in 2012-2013, she still very good. If she carries Moonlight, she can wall huge part of the meta. She has neat ability Levitate, giving her a Ground immunity and ability to her partner to fire Earthquakes freely. If Cresselia carries Rocky Helmet, Ice Beam, and Moonlight, she can actually beat both Kangaskhan and Landorus-T. Kangaskhan is very fastly dead if it hits Cresselia couple of times (and Cress can heal right back with Moonlight), while she can hit Landorus with Ice Beam (landoge cant 2hko cress with literally anything bar maybe banded knock off, while cress can 2HKO or OHKO with heavy sp atk investment).
Those alone makes Cress good. However, her best part is these two: support movepool, and bulk. Cresselia's support movepool is just huge. She is no doubt best Trick Room setter in the game due to fact she is virtually impossible to KO in one turn unless you have powerful STAB super effective moves against her. She can still support alot after TR is up, with moves like Safeguard and Helping Hand.
Cresselia works also as speed controller. She gets BOTH T-wave and Icy Wind, which is cool to choice between what you need. Like i said earlier, it gets Helping Hand, and she is one of the best users of this move due to her massive bulk and her main flaw that she cant elsewhere bring too much offensive pressure. She is one of few users of recovery moves, in this case, Moonlight. In sun, its absolutely nuicanse, healing 2/3 HP back, which can be especially frustating for opponent due to her gigantic 120 / 120 / 130 bulk. She still has huge range of other support moves: Skill Swap, Rain Dance, Sunny Day, and Screens. Cresselia also has decent offensive movepool, having cool moves like Energy Ball and Moonblast, but it really doesnt have space for them.
She even can go offensive: with either Calm Mind or classic ExpertCress. Calm Mind not just doesnt increase her offensive pressure to acceptable levels, but she becomes almost immune to special attacks after couple of boosts (unless crit).
She has so much bulk that she can boost herself with multiple CMs, then heal right back with Moonlight, and then you have monstrous tank, that is nearly impossible to kill. Expert Belt allows Cresselia to bring some surprise value to opponents, making for example sure that she OHKOs most Landorus-Ts, and moves like Moonblast are pretty neat if you use Expert Belt.
And then, this: DAT BULK! Cresselia's bulk is nuicance. Just, look at these:
252+ Atk Parental Bond Mega Kangaskhan Return vs. 252 HP / 156+ Def Cresselia: 97-115 (42.7 - 50.6%) -- 0.1% chance to 2HKO
Avoids virtually always 2HKO from Adamant Kanga's Return. Thats actually crazy amount of bulk considering that she doesnt even resist it or have Sitrus Berry. With Sitrus she can also avoid 2HKO from Double Edge.
252+ Atk Life Orb Bisharp Knock Off (97.5 BP) vs. 252 HP / 156+ Def Cresselia: 159-190 (70 - 83.7%) -- guaranteed 2HKO
Lots of damage, but LO boosted, STAB super effective Knock Off coming from base 125 atk...yeah lol
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 100 SpD Cresselia: 122-146 (53.7 - 64.3%) -- 9.8% chance to 2HKO after Sitrus Berry recovery
If she carries sitrus, she can avoid 2HKO from Aegislash's shadow ball. Base 150 Sp atk, and STAB. Super Effective. Thats some real bulk lol
And then some random calcs:
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 252 HP / 100 SpD Cresselia: 81-96 (35.6 - 42.2%) -- guaranteed 3HKO
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 252 HP / 100 SpD Cresselia: 81-96 (35.6 - 42.2%) -- 0.7% chance to 3HKO after Sitrus Berry recovery
252+ Atk Life Orb Talonflame Brave Bird vs. 252 HP / 156+ Def Cresselia: 74-87 (32.5 - 38.3%) -- 97.6% chance to 3HKO
252+ Atk Landorus-T Knock Off (97.5 BP) vs. 252 HP / 156+ Def Cresselia: 90-108 (39.6 - 47.5%) -- guaranteed 3HKO
252+ Atk Landorus-T U-turn vs. 252 HP / 156+ Def Cresselia: 66-78 (29 - 34.3%) -- 1.9% chance to 3HKO
By those points i listed, Cress sounds truly S rank material. But she still has her flaws.
Cresselia is insanely bulky, but her offensive pressure is meh unless it runs Expert Belt. It can be ignored this way easily (not always though, because potential speed control moves). Cresselia's damage output is overall mediocre. Calm Mind can be stopped with Taunt, and Cresselia in general can be stopped with taunt fairly easily (well, she can still use Icy Wind). Suicune's recent increase in popularity doesnt make things very good either, especially when more offensive teams typically prefers to have cune has speed controller instead of cress. Buffed Knock Off and Dark types buffed in general due to Steel losing Dark resist is also bad thing for Cresselia. Bisharp walls Cresselia, hits her very hard, Icy Wind activates Defiant, and hits her hard with Knock Off. And Bisharp is top 10 in usage, and on so many teams. Aegislash doesnt care virtually anything about what Cress does, so it can use her as setup fodder with Substitute, wall her hard, and hit her hard with base 150 Sp Atk and Ghost typing. Psychic Type is generally not so good either, having only 2 resists, itself and Fighting.
So, rather big post, maybe wasnt necessary, but sharing my opinion. Cresselia would be S rank material if Bisharp/Aegi/Gengar didnt exist and if her sp atk were little higher. She has notable flaws that i listed, but so many good things, so i nominate her rise to A+.