When I read the list of ranking mons, I was really surprised that a lot of them hasn't been already mentioned.
There is a really large array of mons usable in OU, I'll say that Politoed, Smeargle, Starmie, MBlastoise, Espeon, Breloom, Cofagrigus, Sableye, Jirachi, Donphan, Celebi, Salamence, Noivern, MGardevoir, Quagsire, Tangrowth, Kingdra, Florges, Abomasnow, Volcarona, all of them have a place in this metagame. But, as I'm not able to describe 20 pokemons in a same post, I'll focus on 5 of them.
First of all, Mega-Blastoise is, to my mind, simply the best Spinner in the game, as he is able to face absolutely all the commons Anti-Spinners and defeat them, thanks to a boosted Dark pulse, and he's the only one able to do such a thing. Gengar, Aegi, Trevenant, Gourgeist, none of them can switch-in on Mega-Blastoise, so he can make a free spin. Particularly, trevenant can defeat nearly all the other spinners, and Mega-Blastoise is the only one who can OHKO him. Moreover, he also puts somme offensive pressure, because I don't know a not water type-immune pokemon who likes taking a boosted Water pulse or an hydo Pump in his face, before a more adapted move (aurasphere, dark pulse). And, even if Blastoise speed isn't really good, his bulk offsets a little. So, I'm not really good at ranking, because it's the first time I do this, but I think MBlastoise is as good or better than Tentacruel, so I'll say at least B rank, more B+ to my mind.
Then, another really good spinner, and better than Tenta : Starmie. He has always been a top tiers, as he can keep an offensive pressure and spin in the same time. Moreover, it has a reliable healing move : recover, and a really good ability in natural cure, what Tentacruel misses. So definitively, A Rank for Starmie.
Now, let me tell you about Politoed. It's the third Water-type pokemon I'm explaining, not that I love the Water-type, but it seems like you forgot it. May be Rain team aren't viable in this new metagame. That's also my opinion. However, I think it's a playstyle that won't disappear, and perish song on Politoed helps a lot. Because it gives you a free turn then to set up with a rain sweeper, and 5 turn of sweep isn't that simple to handle (admitting the rain is set up for 8 turns). But, even if you think that the rain can not be used by this way, you can't say that a Hydro pump from a specs Toed (so 3 times STABBed) doesn't afraid you. So for him, I think that something like C Rank is adapted.
Another Pokemon isn't here, although his little sister is present. I mean Florges. To me, he's better than Sylveon. More Speed, more Special defense, more Special attack, a little more Defense. The only stat where Sylveon beats Florges is the HP, but I don't think it's enough (I may be wrong). I see you saying : but pixilate makes hyper voice stronger, so he has more Sp atk. I don't know how it learns hyper voice, as it's not on Serebii, however it can learn it on Showdown, but when you are a defensive pokemon, the aim isn't to sweep, but to stall. And Moonblast is better, because it can lower the Sp Atk, to stall better than it already does. That's all. And, definitvely, I think the low speed of Sylveon makes it, mmmh, not dangerous? So, Florges : B rank.
Finally, I want to tell you about a pokemon that isn't the best pokemon ever, but a pokemon I really like, and that has some good points in this new Generation. I mean Tangrowth.
Why 6G Tangrowth is not the same as 5G Tangrowth ? Because of Assault Vest, of course. When you think about him, you think about its incredible defense, and the really good Atk, Sp atk and HP stats it has. But you also think about the extraordinary ability it has : regenerator. So, only the speed and the Spe. Def make him less usable. The former isn't that important with a staller, and the second can be boosted thanks to AV.
Now, if you wonder what are the drawbacks of the Assault Vest : for a staller, it's vital to have a healing move, and the AV forbids it to you. With Tangrowth, there isn't such a problem : Giga-Drain allows him to heal himself by doing good damage (110 Sp atk is pretty good). And, besides, Regenerator heals you another time. Isn't that incredible ? Moreover, it has a quite good movepool, not the best but a quite good one. Acces to Giga-drain, Focus Blast, Earthquake, Rock slide, Sludge Bomb, Power Whip, Knock off, and HP fire generally for coverage, is very good. I hope this is enough to convince you Tangrowth has some great abilities in OU. As I really love it, I think B- rank is the Tangrowth's place.
Also, think about all the other mons I mentioned at the beginnig, I really think they have some place in the Ranking tier list.
