Bringing up a whole new conversation here. I think it was earlier but I'd like to take another shot at this.
Crobat.
In order to be S-tier- "Reserved for Pokemon who can sweep or wall significant portions of the metagame with little support, and Pokemon who can support other Pokemon with very little opportunity cost ("free turns"). Also the home of Pokemon who can easily perform multiple roles effectively, increasing their versatility and unpredictability. If the Pokemon in this rank have any flaws, those flaws are thoroughly mitigated by their substantial strengths. If there are suspects, they will come from this rank."
Crobat cannot sweep or wall significant portions of the metagame. Anything with Scald, Electric types, Steel types with some form of attacking move, Rock types, scarfed offensive threats, anything with a powerful Ice Shard, and anything that isn't 2HKO'd by a 90 base attack Brave Bird really does not mind Crobat. They are neither killed nor are they walled by Crobat.
While it cannot sweep or wall significant portions of the metagame, it is certainly the best anti-metagame Pokemon in the tier. It reliably checks many of the metagames top threats such as Heracross, Roserade, and Mienshao, three of the Top 5 most used Pokemon. It also is a very solid stallbreaker thanks to Taunt and either Toxic or Super Fang, and because it is so good at stallbreaking, most things need Zapdos to check Crobat. When I think of a Pokemon whom I need a check to in order to be successful, Crobat is one who comes to mind.
Crobat doesn't do a huge job at supporting other pokemon with little or no cost. Sure, it can switch in on things that don't like Crobat switching in on them, but anything on the list above can just come in on Crobat and it's running away again, which can hurt if Stealth Rocks are up. It doesn't have a support movepool- they only way it truly supports its team is through momentum, but all it takes is a smart prediction on your opponent's side to ruin momentum.
Crobat's presence alone forces some threats, like the previously mentioned Heraboss, Roserade, and Mienshao, to predict, and that can be costly. Crobat also neuters stall teams should they not have Zapdos, and if you're having trouble with stall, Crobat is a great weapon against it. It can do its anti-meta job with little to no cost.
Crobat cannot perform multiple roles effectively. When you see a Crobat, it has Brave Bird/U-Turn/Roost/Taunt/Super Fang, with an occasional Toxic and Cross Poison. That's it. If I can give you every move you'll ever see competitively and there's fewer than 10, there's not variety. Especially when they don't concern different sets. For example, Kingdra more or less has 8-10 moves you'll see, but there are two, individual, dangerous and powerful sets to worry about when a Kingdra switches in. Three, if you like rain. Crobat pretty much has different options for the same simple goal.
While Crobat cannot perform "many" roles effectively, it can surely do its main set exceptionally well. Crobat's niche as an anti-metagame Pokemon is one that no other can do, and Crobat's unrivaled speed tier along with great typing and U-turn sure complement its role. U-turn even allows it to keep up the momentum. A CB set makes for a fine late game cleaner too. So yeah.
And as far as flaws are concerned... Crobat is SR weak. It's not powerful enough nor does it have the movepool to be used offensively, and it's not bulky enough nor does it have the perfect movepool to be used as a wall. You don't just slap a Crobat on a team and call it your physical wall, you use it for stopping certain threats. And sure, its band set does some damage. But you can slap a band on Hitmontop or Nidoking and deal more damage. Not to mention it truly does have issues switching in on attacks- stealth rocks + anything non-resisted will bring Crobat very close to death, and any form of revenge killing will pretty much be the end of it.
Crobat's flaws are mitigated by qualities. (I'm out of time so I'll finish up this argument later.)
No, I'm not arguing that it's bad. It's pretty useful and nifty when given the right opportunities. But it's not S. It's A. It doesn't sweep most of the tier, it doesn't wall most of the tier. It doesn't have variation, and the significant flaws (of being totally walled by every steel type ever, weak to ice when hail is flying around, and having the SR weakness) are not made up for by anything in particular.
Plus, unexpected scarves also exist. Scarfed Raikou, Zapdos, Weavile, Azelf, and a few other anti-scarfers can spell the end for Crobat in a very short amount of time.