Oiawesome, you're seriously underestimating Cresselia's ridiculous bulk. Having a "strong Dark or Ghost move" isn't in any way enough preparation to handle Cresselia most of the time, and of the things that you claimed "beat" it, depending on its set Cresselia is perfectly capable of either beating or setting up 50/50 situations with either of them. Anyways, onto those calculations that I mentioned.
Looking at those calculations, it's pretty clear that even some of the best Cresselia "counters" out there are actually incapable of handling it all that well. The only Pokemon on that list that are capable of beating Cresselia perfectly every time are Taunt Drapion and Taunt Skuntank (who still lose to random CM HP Ground sets and stuff, but not going to take that into account). Cresselia can either Toxic or just boost up against CB Pursuit-locked Pokemon, as most of them are 5HKOing at best. The only Spiritomb that's consistently beating Cresselia is CroTomb.
But the biggest qualm that I have with your post is that you state that Cresselia didn't centralize the metagame, which is just flat out untrue. It shifted the entire metagame towards Bug/Ghost/Dark types (although admittedly, the dropping of Escavelier somewhat helped with this). And I'm not sure where you're coming from about Cresselia not walling the vast majority of the tier - there are literally three notable Pokemon in RU that are capable of hitting Cresselia hard enough to leave a significant dent, and none of them can switch in on certain moves without being KOed or crippled for the remainder of the match. It's a ridiculously dominant defensive force, and it really needs to go.