Sonic 3 A.I.R

An unofficial remake of Sonic 3 & Knuckles, with widescreen support and extra tweaks, making this essentially the definitive version.

This is one of the only fangames that work on iOS, through Safari web apps.

https://sonic3air.org/

Sonic Triple Trouble 16-Bit

An unofficial remake of the game Sonic Triple Trouble for the Sega Game Gear in the Sega Genesis's 16-Bit style. Adds more characters, changes the level design, ultimately making it the definitive version of the game.

https://gamejolt.com/games/sonictripletrouble16bit/322794

Sonic 1, 2, CD, and Mania RSDK Decompilations

With fan efforts, the code for the Sonic 1, 2, CD, mobile ports + Sonic Mania have essentially been recreated, meaning they can be ported to any platform.

https://github.com/RSDKModding/RSDKv5-Decompilation

Sonic the Hedgehog Forever

A mod of the Sonic the Hedgehog 1 RSDK Decompilation, adding new menus, options, and characters to make the definitive version of Sonic 1 by Team Forever.

https://teamforeveronline.wixsite.com/home/sonic-1-forever

Sonic the Hedgehog 2 Absolute

A mod of the Sonic the Hedgehog 2 RSDK Decompilation, adding new menus and options to make the definitive version of Sonic 2 by Team Forever.

https://teamforeveronline.wixsite.com/home/sonic-2-absolute

Sonic 1 SMS Remake

A remake of the Master System/Game Gear version of Sonic the Hedgehog 1. Adds widescreen support, new characters, and options, making this the definitive version of Sonic 1 8-Bit.

https://sonic-sms-remake.blogspot.com/p/one.html

(This website also hosts many other Sonic fangames made with the same SMS engine)

Sonic 2 SMS Remake

A remake of the Master System/Game Gear version of Sonic the Hedgehog 2. Adds widescreen support, new characters, and options, making this the definitive version of Sonic 2 8-Bit.

https://sonic-sms-remake.blogspot.com/p/two.html

Sonic Robo Blast 2

A Sonic fangame created in the DOOM engine.

https://www.srb2.org/

Emulators

Not specifically a Sonic game, but emulators can be used to emulate Sonic games (or any game) from older game platforms.

RetroArch can be used to emulate games from the Atari 2600 to the GameCube or PlayStation 2.

I would recommend Dolphin to emulate GameCube and Wii games, NetherSX2 for Playstation 2 games, Citra for Nintendo 3DS games, and Yuzu or Ryujinx for Nintendo Switch games.

Most of these are only available on Android, but RetroArch and Delta are available on iOS.