here’s a quick list of some stand-out Sonic ROMhacks as someone who has played a lot of them:
Sonic Megamix - requires SEGA-CD. It’s not a Sonic CD romhack, it’s a Sonic 1 one. Might actually be playable without the SEGA-CD, actually? It was a huge community project that got canned, so the hack is incomplete but it is incredibly high quality. Features Sonic, Shadow, Mighty, Tails and Knuckles and each has an expanded moveset of their own.
Sonic 1 Published by EA - a joke ROMhack, I won’t spoil more than that
Sonic 1 Point & Click Edition - Sonic 1 but you can play it with the Mega Drive mouse add-on and the mouse functionality is honestly incredible. You can interact with almost anything with the cursor.
SNOLF - Sonic golfing. There’s also SNOLF 3 & KNOLF which is based on Sonic 3 & Knuckles
Metal Sonic Rebooted - high quality hack in which you play as Metal Sonic on all new levels and somewhat new bosses, might as well be a new game altogether
Lose Tails or Else - Sonic 2 except you have 30 seconds to despawn or kill your partner Tails or else you die. Once you meet that requirement you instantly go to the next level. This means that it’s really fast-paced and you can finish the whole thing in a few minutes.
Yoshi in Sonic 2 - Yoshi from Yoshi’s Island in Sonic 2 with all of his mechanics intact. Instead of Baby Mario you have Baby Sonic. Rings act as your timer for when Baby Sonic is in danger.
Sonic 1 Omochao Edition - It’s Sonic 1 but Omochao from Sonic Adventure 2 is there to help you
Sonic 2 Retro Remix - Very high-quality. Levels are designed like Sonic levels, but they also have Super Mario 64-style objectives for you to clear so you can get Emeralds (SM64 Stars) to unlock other levels. Some Emeralds require exploring the level to find them hidden in there, while others are time and score attack based - very good score system. Sonic has an expanded moveset that is explored in interesting ways
Sonic Classic Heroes - You play as Sonic, Tails and Knuckles at the same time going through the entirety of Sonic 1 and 2 in the same game, featuring both Special Stages in every level
Pana der Hejhog - Small high quality romhack. Features new level gimmicks, a new boss, and double the frames of animation, before Sonic Mania went and did that.
Sonic VR - A bunch of small laser-focused technical challenges. More Super Meat Boy-esque than Kaizo-esque.
An Ordinary Sonic Romhack - A totally normal non-cursed version of Sonic 1
I could list more but those are the only ones that I’m completely sure about recommending, other ones would require me to go play them to remember what they were about or whether they really are good or not.