I am just saying that Cresselia is fine to me, but its recovery is a small bit lackluster to me. Having 8 PP Moonlight is like, not great, seeing Toxic will not wear opponents down in 8-turns. Recover is way more reliable (more PP, not effected by the /rare/ abomasnow) and Trace is actually better against Mence / Krook 1v1, because you Trace intimidate and lower their Attack. Furthermore, P2 has slightly superior offensive presence
0 SpA Porygon2 Ice Beam vs. 0 HP / 4 SpD Krookodile: 180-214 (54.3 - 64.6%) -- guaranteed 2HKO
0 SpA Cresselia Ice Beam vs. 0 HP / 4 SpD Krookodile: 136-162 (41 - 48.9%) -- guaranteed 3HKO
Cresselia: 63.5 - 74.7% + 42.3 - 50.4% = Guaranteed 2HKO
Poryogn2: 30.4 - 36% + 30.7 - 36.3% = not ever a 2HKO, it at times even avoids the 3HKO.
While the difference is rather small, in this case, Krookodile isn't 2HKOed by Cresselia, which means it gotta take a lot of damaage from Knock off, which 2hkoes, and it is forced to switch after or recover. While Porygon2 2HKOs only, if you hit it, Krook will be locked into Knock off and since it isn't SE, Porygon2 will be able to take the next one while regaining some damage to Recover until it has enough HP.
So, Porygon2 is slightly bulkier and has superior offensive presence. The only benefit Cresselia has that it is less item-reliant, a small bit better typing, and to an extent, Levitate? Trace and levitate both have their advantages.
Regardless, I think P2 belongs in B rank.