Skuntank can stay right where it is. Sure, it's good at checking Gardevoir and Musharna, but that's all the Skunk has got going for it. It's a revenge killer and all, and that's it. It's not amazing.
Also, I second Gardevoir for S-Rank. Let's take a look at the definition.
"Reserved for Pokemon who are amazing in the NU metagame."
Gardevoir is amazing. No duh.
"These Pokemon are usually able to perform a variety of roles effectively, or can just do one extremely well."
Gardevoir can fill a lot of roles thanks to her vast movepool, almost as big as that of Smeargle/Mew/Absol. This, combined with her high overall stats, gives her the ability to do many things very effectively. She can be a powerful offensive Psychic-type, dealing heavy damage to the opposition. A support set is also possible with that high Special Defense, and Will-O-Wisp can burn physical attackers like Sawk and friends. She's can also serve as a WishPasser and cleric, despite that rather depressing HP stat, although that does compete with Audino. Dual Screens, of course, is possible too. Then there's also weather support, if you really want it, and Choice sets. Gardy can run many different sets, all can be used somewhat effectively.
"Their use has low risk involved and high reward exerted."
I don't see much risk in using Gardevoir at all. With such a great movepool and good stats to boot, it's pretty easy to use and very versatile, and also a powerful Pokemon. With all that, there's much reward exerted, as Gardy will surely do a great job and won't fail to impress. A great sweeper too.
"Pokemon in this tier have very few flaws that are patched up by numerous positive traits."
This thing has only few flaws.
- Pursuit weakness
- Pisspoor Defense
- Meh Speed, albeit decent for NU
- Vast movepool, even allowing it to deal with Skunk with Will-O-Wisp, etc
- High SpD, can burn physical attackers, patching up that poor Defense modestly
- Hits hard
Overall, Gardevoir is a great and versatile Pokemon in RU, possibly one of the best in NU.
Gardevoir qualifies for S-Rank.