My opinion, keep in mind I've played for less than a month and this is just through playing/watching tour games so I wouldn't put much if any weight on it.
Beyond this I don't have enough knowledge to say more.
S Rank:
Mega Blaziken to A+: Think Blaziken is a fanstastic mon, however, I don't believe it is in the same tier as mimikyu/landorus-t/mega mence in either versatility or pure power. For me, the above three can be stuck on the vast majority of teams and put in work. A mimikyu is probably getting you a kill or close to it almost every game if you play it right, mence in my opinion is the most dangerous set up sweeper without significant support in the game, only being compared to gyara under screens, lando can do practically anything you want it to. The most important thing is, I think it struggles, for the most part against the three that share its current tier as well as quite a few mons in particularly the A tier, making it more difficult to slap blaziken on teams.
A+ Rank:
Blaziken to A: Just a, for the most part, worse version of mega blaziken. Yes, doesn't require a mega slot but that's the majortiy of the benefit.
Gyarados to A: I've been debating this one to myself. Gyara in the right circumstances can wreck havoc. I love the fact that regular and mega cover the fighting weakness. I think though, Gyara is best under screens and screens as a playstyle is finicky from my experience of trying. The spdef stat and ability not to fall over to ice moves and the fact regular gyara > regular mence gives gyara a niche vs mence, but the majority of teams I think mence is just better.
A Rank:
Looks pretty solid to me.
A- Rank:
Kartana to A: I think Kartana is actually underrated and could potentially move up to A. It's one of the scariest scarfers and in general mons to have in the back in my opinion.
Cloyster to B+: The problem with Cloyster is it needs to run sash, easily brought down to sash and easily revenged. Generally most mons can pressure it enough before shell smash that it either gets only 1 kill or if the right mon is in, zero.
B+ Rank:
Glalie to B: I just don't think it is as good as people make it out to be. Annoying yes, RNG based yes, but on its own, a lot of mons actually deal with it well unless you let it get the sub and the evasion boost etc.
Gengar to ???: Wherever it belongs, it isn't in B+. I'd be tempted to say its mega is overrated, but gengar itself has no place here.
Pheromosa to B: Another mon that I think is falling off. No doubt busted in OU, but doesn't quite do enough damage and Mimikyu/Aegi which are very common bop it unfortunately.
B Rank:
Kingdra to B-: I think Kingdra is a great rain mon in 6v6, but imo swampert is your rain abuser to have in BSS. I don't think rain is the strongest style by any means anyways.
Politoad to B-: I think for most purposes, Pelipper outclasses it.
Quagsire to B-: haven't actually seen it much around, not even on stall.
Ryperior to C+: It just isn't that good from what Ive seen.
B- Rank:
Altaria to C+: I don't think Mega Altaria from the few times I've faced against it is anywhere near as threatening as the mons in this bracket.
Mega Blaziken to A+: Think Blaziken is a fanstastic mon, however, I don't believe it is in the same tier as mimikyu/landorus-t/mega mence in either versatility or pure power. For me, the above three can be stuck on the vast majority of teams and put in work. A mimikyu is probably getting you a kill or close to it almost every game if you play it right, mence in my opinion is the most dangerous set up sweeper without significant support in the game, only being compared to gyara under screens, lando can do practically anything you want it to. The most important thing is, I think it struggles, for the most part against the three that share its current tier as well as quite a few mons in particularly the A tier, making it more difficult to slap blaziken on teams.
A+ Rank:
Blaziken to A: Just a, for the most part, worse version of mega blaziken. Yes, doesn't require a mega slot but that's the majortiy of the benefit.
Gyarados to A: I've been debating this one to myself. Gyara in the right circumstances can wreck havoc. I love the fact that regular and mega cover the fighting weakness. I think though, Gyara is best under screens and screens as a playstyle is finicky from my experience of trying. The spdef stat and ability not to fall over to ice moves and the fact regular gyara > regular mence gives gyara a niche vs mence, but the majority of teams I think mence is just better.
A Rank:
Looks pretty solid to me.
A- Rank:
Kartana to A: I think Kartana is actually underrated and could potentially move up to A. It's one of the scariest scarfers and in general mons to have in the back in my opinion.
Cloyster to B+: The problem with Cloyster is it needs to run sash, easily brought down to sash and easily revenged. Generally most mons can pressure it enough before shell smash that it either gets only 1 kill or if the right mon is in, zero.
B+ Rank:
Glalie to B: I just don't think it is as good as people make it out to be. Annoying yes, RNG based yes, but on its own, a lot of mons actually deal with it well unless you let it get the sub and the evasion boost etc.
Gengar to ???: Wherever it belongs, it isn't in B+. I'd be tempted to say its mega is overrated, but gengar itself has no place here.
Pheromosa to B: Another mon that I think is falling off. No doubt busted in OU, but doesn't quite do enough damage and Mimikyu/Aegi which are very common bop it unfortunately.
B Rank:
Kingdra to B-: I think Kingdra is a great rain mon in 6v6, but imo swampert is your rain abuser to have in BSS. I don't think rain is the strongest style by any means anyways.
Politoad to B-: I think for most purposes, Pelipper outclasses it.
Quagsire to B-: haven't actually seen it much around, not even on stall.
Ryperior to C+: It just isn't that good from what Ive seen.
B- Rank:
Altaria to C+: I don't think Mega Altaria from the few times I've faced against it is anywhere near as threatening as the mons in this bracket.
Beyond this I don't have enough knowledge to say more.