One thing quickly is I would disagree with aegi ->4. It's sub set is very solid, and pairs incredibly well with Bul. If you play king shield right it can completely destroy physical attackers, and it's the best ghost stab now that Gengar is gone.>>Tier 2<<
Pokemon that are generally strong and can easily be placed on a variety of teams, but don't have the same level of prowess as the threats in Tier 1.
Mega Charizard-Y --> stay in tier 2
Zard-y teams are strong, but I don't think zard should be tier 1 for two main reasons. First, I don't think Zard carries its teams enough to be tier 1. In tier 1 we see mons like Landorus, Incin, Fini, mons that can be on almost any team in the game, regardless of any of the other pokemon. Metagross fits onto almost any core and is one of the most broken Megas in the game. Charizard doesn't reach that level of prominence. Charizard teams are strong, yes (I've played pretty much only one team since snake and it's a zard team), but that's what they are. They're charizard teams. Charizard does the vast majority of the work in those games, and if it doesn't, I lose. I think the fact that charizard has to so much be *built around* means that it ought not be tier 1.
Tapu Bulu --> tier 3
Bulu is outclassed by other grass types and generally makes your team pretty weak. It hung on to top tier relevance due to the threat of scarf bulu + Gengar, but I think with Gengar gone that bulu is going to decline in usability greatly. The mon is just worse than kartana and amoonguss, and with koko coming back into relevance, isn't needed as much to check fini. Let bulu go to tier 3, where it belongs.
Tapu Koko --> stay in tier 2
Not super convinced on this one, but I think that Koko just isn't as good as the other tier 1 mons. Would like to wait on noms to tier 1 until after koko shows its strength in SPL
Zygarde --> tier 1
Zygarde is an effective offensive replacement to landorus on many teams. Currently landorus is mostly used as a bulky pivot, using berry and stealth rocks and u-turn to facilitate intimidate shuffling, and sometimes it does damage with earth power. While those are all strengths big enough to make lando tier 1, Zygarde provides the offensive ground damage that a lot of teams would like, without the harm of side eq. Zygarde's band set has been getting a lot of traction, especially with that team that Stax made to go 40-0 in dlt. I think that Zygarde, while it doesn't have all the supporting tools, is a top tier threat that fits on a lot of teams, as incineroar can often fill the intimidate slot sufficiently, leaving zygarde to do the damage.
Pokemon that are generally strong, but can be dead weight in some matchups, don't have great matchups vs a lot of Tier 1 and 2, or are only particularly useful for checking a certain team style.
Aegislash --> tier 4 or lower
Sord sucks right now I know you guys know this everybody knows this. Earth power, incineroar, and z moves have all combined to make this pokemon that was a defensive and offensive wall, merely a slow, high damage mon that doesn't even work under trick room. Gengar leaving makes aegislash a bit stronger potentially, but I don't think it should have been tier 3 in the first place.
Ferrothorn --> tier 4
Ferrothorn sucks and is not generally strong, the other grass types (amoonguss kartana bulu) are far stronger. Ferro only really gets use on rain teams, and barely even then. Incineroar's existence makes this mon pretty much worthless. Dropping it one tier seems fair considering its usage.
Stakataka --> tier 4
Stak is not generally strong. The quad weaknesses to fighting and ground, along with the stunningly low speed and weakness to taunt leave stak high and dry a lot of the time. It only sees use on hard trick room, an archetype that is strong, but not overpowering. Considering that the pokemon that actually makes hard trick room good, camerupt, is tier 4, and stak only sees use with camerupt specifically, I think dropping it down to be buddies with camel makes sense. Stak is droppable on that team and camel is not, and stak sees no use elsewhere.
Suicune -->tier 4
If you want tailwind, run zapdos, if you want a water type, run fini. Suicune doesn't do damage and gets hurt by the fini hate that teams have to run. I rarely if ever see suicune at high level competitive play, and the role compression it offers doesn't seem like enough to have it be tier 3.
Pokemon which have broad applications on a variety of teams but are simply less effective than the Pokemon in the higher tiers. This also includes Pokemon which, while good, only fit on a specific team style or require heavy support but are still stronger than Pokemon in the tiers below.
Celesteela --> tier 5
I literally have not seen this mon once in 6 months. It suffers from the ferrothorn problem. Every team has a fire type, or an electric type, or both. While ferrothorn has the rain niche, celes does not. Is celes generally stronger than mega latias, heatran, lando-i, araquanid, lurantis and naganadel, all of which have seen recent snake success? I don't think so.
Chansey --> tier 3
Chansey's niche of being a special wall has been invaluable in testing and in tournament play. Counterplay exists to chansey to be sure: physical mons, knock off, ohko'ing the partner. However, chansey's special bulk puts a ton of pressure on opponents at all points throughout the game, and provides a wincon that almost no other pokemon in the game does right now. With gengar and its immunity to seismic toss leaving DOU, chansey only gets stronger.
Mega Salamence --> tier 4
Mence blows @bowman. With koko getting better, mega man getting better, double intimidate and/or fini on every team, I'm really not sure mence has much to go for it in the current meta. Not to be rude, but iirc your snake teams with mence on them didn't do well either, giving it not only a bad performance in usage, but a bad performance in tournament play as well.
Cresselia --> tier 4 or 3
Cress has done great in snake and is the backbone of most semi-room teams. the combination of levitate, massive bulk, and ice coverage has led to it being the TR'er of choice for teams that are slow, but not too slow. I think that fits the definition of 'This also includes Pokemon which, while good, only fit on a specific team style' for tier 4