They are. They rebranded Microsoft Studios as Xbox Game Studios in 2019. Microsoft wants their gaming division to be synonymous with the Xbox brand. 

Top tier: Bethesda, Playground, Mojang, id, Ninja Theory, Rare. Based on a mix of game quality and sales.

Almost all developers there are at least good though.

Well, now what Activision-Blizzard is being acquired, I guess I will give a tier list here:

S Tier (top tier studios/teams, their games are well received critically (85+), well received commercially (pretty big sellers), and I personally have played every game they have released in recent years and am interested in playing all future games from them)

• Bethesda Game Studios
• Playground Games
• 343 Industries
• Blizzard team 4 (Overwatch series)

A tier (studios/teams that meet at least 2 of the 3 criteria above or the other, but not all 3)

• The Coalition
• id Software
• MachineGames
• Arkane Lyon
• Arkane Austin
• Tango Gameworks
• Turn 10
• Blizzard Team 1 (Starcaft series, Heroes of the Storm, Warcraft 3 Reforged)
• Blizzard team 2 (WoW)
• Blizzard team 3 (Diablo series)
• Blizzard team 5 (Hearthstone)
• Obsidian Entertainment
• Infinity Ward

B tier (studios/teams that meet 1 of the criteria above, but not 2)

• inXile
• Treyarch
• Sledgehammer Games
• Rare Ltd.
• Mojang
• Double Fine
• Undead Labs
• Zenimax Online Studios

C tier studios (studios that fail to meet any of the 3 above criteria currently, but I see S, A, or B tier potential for in the future)

• The Initiative
• Compulsion Games
• Roundhouse Studios

D tier (support studios currently, though there is potential to go higher for some of them in the future, based on past experience)

• Beenox
• Toys For Bob
• Demonware
• High Moon Studios
• Alpha Dog Games

F tier (studios I have no interest in whatsoever, mobile developers)

• King
• Digital Legends Entertainment
• Alpha Dog Games
• Solid State

Whats an s tier studio?

AAAA studio. Get with the program dude. Its when you level up from AAA.

https://tiermaker.com/blog/news/15/s-tier-meaning-what-does-s-tier-stand-for

S-rank originated from academic grading in Japan. Teachers could use a grade of S to showcase a level superlative to grades such as A, B or C. A way to incentivize truly exceptional academic performance and give recognition to elite students earning near perfect scores.

343 Industries an s tiers, what is rockstar then , an sss tier studio?

S Tier

Blizzard

The Coalition 

Bethesda

Playground games

A tier (some of these could become S tier in the future)

iD Software

Arkane

Obsidian

Ninja Theory

Treyarch

Infinity Ward

B Tier

Double Fine

Toys for Bob

Inexile

Machine Games

Tango Gameworks

Beenox

Turn 10

C Tier

Undead Labs

Radical Entertainment

Mojang

Rare

Compulsion Games

The Initative 

Worlds Edge

Zenimax Online

Raven software

High Moon Studios

D tier

Alpha Dog

Sledgehammer

Roundhouse

Demonware

King

343 Industries

I probably left out some studios but I think I listed all the relevant ones. I'd probably say anything B tier and higher are the good to great studios while the C tier is largely studios who have either produced very little in recent times or are newly formed and haven't released anything. 

No they are an Asss tier studio its when you level up again beyond S tier and rip your users up the ass. Trust me it gets way more complicated then when we had A,AA,AAA studios in the past like i said its the new age now.

Rockstar ripped your ass?