Next, Rotom-Wash for S rank.
Rotom-W is one of the best Pokemon in the game right now. It's not perfect, but very close to being flawless, IMO.
As has been stated previously, it has next to no opportunity cost at all, fits onto almost any team, requires very little support, supports its teammates with uncommon STAB types (good Electric STAB is kinda rare in OU), boasts excellent defenses and resistances that allow it to stop many common threats without any specialization of its set, and
A bulky Water-type with Electric STAB, Levitate Will-O-Wisp, Pain Split, and Trick is extremely odd (flavor-wise), and when you think about it, a combination of all of the best traits a bulky Water could have. WoW, Hydro Pump and Volt Switch is an extremely potent combination, and makes it very easy for Rotom to gain the upper hand. Volt Switch provides excellent momentum, and if you've ever accidentally made a team that's Rotom-weak, you'll know how hair-tearingly insurmountable this move can be. STAB Hydro Pump is excellent against nearly every mon that can block Volt Switch, and WoW is WoW. If given a free turn, Rotom almost always does something prouctive, and gives you some sort of advantage.
And Rotom will get those free turns. This thing's resistance list is insane, and can form an effective core with a lot of good mons. Its Volt Switching nature and pairing inclination mean that it works best with the help of other mons, but this in no way means that it requires support to function, which means that it does qualify for S rank in my eyes.
One standout thing is that, no matter which of its sets you choose to run, it'll always be able to do the same things. It's always a great Talonflame check. It's always a good VoltTurner. It's always a good check to bulky Ground types and (some) physical attackers. As a tank, it lacks reliable recovery. Anyone can tell you that this hardly matters. Rotom uses Pain Split excellently, and the RestoChesto set is a nightmare. The most common way to deal with Rotom-W is to wear it down. A single free Rest shouldn't be so effective, but it is. The fourth slot gives it a measure of unpredictability, and can make it even harder to deal with. TrickScarf can screw over your walls and offensive Pokemon alike, and, unlike some other Trickers, doesn't render Rotom deadweight after the item swap.
S Rank: 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.
Rotom-W fits this to a T.
So what are Rotom-W's flaws?
When you think about it, there aren't any, really. You could say that it has a Grass weakness, or its Special Attack isn't as high as most special attackers, but every Pokemon has weaknesses, and Rotom-W isn't a sweeper. Its base Speed is bad for an offensive Pokemon, but good for a tank. With a Scarf and Modest nature, it outspeeds every relevant unboosted Poke in the tier, barring stuff like Mega Alakazam and Manectric.
Does it have any glaring type weaknesses? No, Grass-type moves are fairly uncommon, and usually found on tanks or as weak coverage moves. Mold Breaker Earthquakes are a threat, but I'd hardly call that a flaw, considering that Excadrill is the only user, and it doesn't like Hydro Pump.
I'm struggling to come up with something here. Most Pokemon have at least one big flaw. Genesect is really good, but its standard sets are walled by Heatran. Mega Vensaur is really good, but it's Talonflame bait. Gengar is really good, but it's frail. Volcarona is really good, but it's weak to Stealth Rock.
Rotom-W is really good, but...