Sonic the Hedgehog (1991)

No in-game story credits.

Naoto Ohshima created Sonic and Eggman. Yuji Naka was also presumably involved in the creative process.
[Source?]

Sonic the Hedgehog 2

Game credits

Game manual: Youichi Takahashi, Carol Ann Hanshaw

The latter game manual credit is presumably for the English manual.

Tails was designed before his age was decided. The character designer, Judy Totoya, designed him as if he was 5 years old, however for the final game his age was decided to be 8, which has remained consistent in Japan since.
(Source)

Sonic the Hedgehog CD

???

Sonic the Hedgehog 3

???

Sonic & Knuckles

???

Sonic Adventure

Game credits

Story Event Coordinator: Naoto Ohshima
Story Event Director: Akinori Nishiyama
Scenario Writer: Akinori Nishiyama
Event Scene Editors: Shintaro Hata, Kenichi Fujiwara, Eitaro Toyoda

Naoto Oshima is also known to have storyboarded the first scene in Sonic's story, although the in-game scene is quite different due to technical limits.
(Source)

Jun Senoue and Takahiro Fukada wrote and translated song lyrics, which are used to enrich the narrative.
(Source) Better source needed.

Sonic Adventure 2

Game credits

Scenario Writer: Shiro Maekawa
Story Event Designer: Shiro Maekawa

The story is expanded by the official guide. Writers for the official guide are unknown. Information from the guide was still referenced by Eitaro Toyoda on Sonic Channel in 2016.
(Source)

Sonic Heroes

Game credits

Scenario Writer: Shiro Maekawa
Event Scene Artist: Nobuhiko Honda
Story Board Designers: Shiro Maekawa, Kazuyuki Hoshino

Shadow the Hedgehog

Game credits

Scenario Writer: Takashi Iizuka

Sonic the Hedgehog (2006)

Game credits

Game Story: Shun Nakamura
Game Script: Kiyoko Yoshimura, Shiro Maekawa
Storyboard: Tatsuya Yabuta, Masaya Suzuki

Shun Nakamura asked Shiro Maekawa to write a story similar to Sonic Adventure 2. Shun Nakamura decided that the story should be about a hedgehog with psychic powers who comes from the future, inspired by Trunks from Dragon Ball, that Sonic, Shadow, and Silver should each have their own story. Shiro Maekawa decided the structure of the scenes and wrote key lines. Kiyoko Yoshimura wrote the details.
(Source)

Shiro Maekawa wanted Elise to be named "Oliga", but an "overseas sales person" forced her to have a western name instead of a Russian inspired name.
(Source)

Sonic and the Secret Rings

No in-game story credits.

Shiro Maekawa wrote the initial version of the plot, Yojiro Ogawa rewrote about half of it.
(Source)

Sonic Unleashed

Game credits

Scenario Writer: Kiyoko Yoshimura

Sonic and the Black Knight

No in-game story credits.

Written by Shiro Maekawa, who was more or less allowed to do what he wanted. He was surprised Caliburn was made male when he wanted him to be a girl, which is his main regret with the game.
(Source)

Sonic Colors

Game credits

Story Writers: Ken Pontac, Warren Graff, Yasushi Otake

Ken Pontac said that he joined late in development, when the story and script were already largely decided. Pontac and Graff make changes to the script. The English and Japanese end up with major differences.

Sonic Team wants to get rid of Orbot and Cubot because they think they make Eggman OOC. Pontac and Graff insist on keeping them. (It's unlcear if this refers to Colors, meaning Sonic Team wanted to scrap Orbot and Cubot entirely, or if it refers to Lost World)
(Source)

Sonic Generations

Game credits

Writers, Story and Script: Ken Pontac, Warren Graff

Only Ken Pontac and Warren Graff are credited, but it's unlikely that they were the only contributors. Sonic Team presumably came up with the premise.

Sonic Lost World

Game credits

Writers (Story and Script): Harumasa Nakajima, Ken Pontac, Warren Graff

Sonic Team (presumably Harumasa Nakajima) comes up with the premise. Pontac and Graff are given ideas to write a story and script around, and the do so. Their script is supervised and later adapted for the Japanese version.
(Source)

Sonic Forces

Game credits

Writers, Story and Script: Makoto Goya
Writers, Japanese Story and Script: Eitaro Toyoda
Writers, English Story and Script: Ken Pontac, Warren Graff

If the allegedly leaked drafts of the script are to be believed, the script was initially written in Japanese, then it was translated back and forth between English and Japanese with various people making changes to it, and various other people commenting and giving advice or, in the case of Iizuka, orders on what to change. (Leaked script)

Ian Flynn was tasked with writing the English script, but his work wasn't used because he was late. Ken Pontac and Warren Graff finished it first, so theirs was used.
[Source?]

Sonic Frontiers

Game credits

Writer, Story and Script: Ian Flynn, Morio Kishimoto, Jyunpei Ootsu

Sonic Team comes up with the premise. Ian convinces Sonic Team to make the Ancients ancestors of the Chao.
[Source?]

Ian writes the script for the main scenes. The scenes are animated following Ian's script (noticeable in certain scenes where the Japanese dialogue doesn't match the animations). Kishimoto considers the main script written by Ian unsatisfying for the Japanese/Asian market, so he makes several changes to the main scenes in the Japanese version.
(Source)

Ian Flynn also writes side dialogue (idle commentary and the Egg Memos), which is plainly translated to Japanese without major changes.
[Proof?]

Sonic Frontiers: The Final Horizon

No in-game story credits.

Sonic Team comes up with the story. Ian writes the script for the few main dialogue scenes, which is plainly translated to Japanese without major changes. Unknown developers at Sonic Team write the one-to-one character dialogue, and is plainly translated to English without major changes.
[Source?]
[Proof?]

Shadow Generations

Game credits

Writer, Story and Script: Ian Flynn

Only Ian Flynn is credited, but it's unlikely that he was the only contributor. Sonic Team presumably came up with the premise.

Side Media

Sonic Channel - Sonic Comic

Japanese
Fan Translation
[Official Translation]

Written by Eitaro Toyoda.
(Source)

Sonic Mania Adventures

???

Sonic Forces Digital Comic

Official credits

Story: Sonic Team
Writer: Ian Flynn
Art and Letters: Adam Bryce Thomas

Adam Bryce Thomas wanted Infinite to use a sword. When Sonic Team learned this, they informed him that Infinite already had a sword they had designed for him, and he was allowed to use it in the comic.
[Source?]

Sonic Channel - Cover Stories

Japanese

Written by Eitaro Toyoda.
[Source?]

Sonic Colors Comics

The whisper of invasion - Japanese English
Egg Planet Park - The beginnning of an adventure - Japanese English

These comics were made by Eitaro Toyoda and Yui Karasuno.
(Source)

Sonic Colors: Rise of the Wisps

Directed by Tyson Hesse. Written by GGDG, Tyson Hesse. Japanese script written by Eitaro Toyoda.

Sonic Channel - Wallpaper Ohgiri

Japanese

Written by Eitaro Toyoda.
[Source?]

Sonic Frontiers Prologue: Convergence

Written by Ian Flynn.
(Source)

Sonic Team involvement unknown.

Sonic Frontiers Prologue: Divergence

Directed by Tyson Hesse. Written by Ian Flynn.

Sonic Team involvement unknown.

Sonic Channel - Isekai Ohgiri

Japanese

Written by Eitaro Toyoda.
[Source?]