I agree that Skuntank should remain A-rank
CBT summed it up pretty well. Skuntank is fantastic at removing Psychics (and Toxic Spikes too I guess), but to be honest, aside from revenge killing with Sucker Punch, that's about it. And, at the end of the day, Skuntank doesn't always beat Gardevoir, thanks to the threat of will-o-wisp
The vast majority of NU teams carry a bulky rock type to counter Birds and set down Stealth Rock. These things completely wall Skuntank and can usually take it down in return. As someone else said, Skuntank loses to other pokes that it's not designed to take on. Basically, Skuntank has to fight a battle on its own terms. If it does, it will usually win. If it doesn't, it will usually lose. Stuff like Braviary, Samurott and Sawk, to name but a few, are all common threats that can take on Skuntank with ease, thanks to Skuntank's fairly sub-par offensive stats.
Skuntank is definetly a threat in the NU metagame but in my opinion, it's certainly not worth of a place in the S-rank.
Speaking of which, why is S rank always higher than A rank?
RU: #54 | NU: #15
Ninjas With Guitars?
I always use one as my team's Stealth Rocker.
Ice Cold Fire