SADX Mod Installer - THE certified way to downgrade back the OG PC port and enabling modding. Comes with a whole suite of fixes and patches to match the DC release 1-1. The installer will ask which mods you want, so it's extremely customizable.
SA1 Chao World Extended - - Restores several SA2B Chao world features and offers some QoL to make SA1's Chao Garden worth playing alongside SA2B's.
Physics Swap Mod - Allows you to customize the physics of every controllable character. I'd recommend this just to swap Amy's physics to Sonic's: Believe me, it really isn't worth playing with her OG physics, as they were absolutely dreadful in every release of the game. Swapping physics is done in the mod's config editor in the mod loader!
ESRGAN-AI HD Textures - HD textures. May not want to run if you don't think your PC can handle them, though. The installation has an extra step than just popping it in the Mod Loader, so be sure to read the description.
HD Rings - Pretty Rings!
HD Shadows - Pretty shadows!
DX Characters Refined - DX models that don't look like plastic!
DX-consistent models - For some reason, back on the OG release of DX, Sega half-baked the model upgrades and just flat out didn't make new models for a buncha NPCs. This mod attempts to fix that by adding more DX styled models in! Finally, Eggman doesn't look funky anymore.
High-Poly Gamma - Sega cheaped out on giving best boi a model upgrade, for some reason. This fixes that and makes sure he doesn't look weird next to the rest of the DX cast if you use the DX models.
HD Objects - Pretty objects!
Disable Upgrade models - Disables the upgrade models that you would get for things like Sonic's bracelets, Tails' Badge, Knuckles weird yellow gloves, etc. 100% NEEDED IF YOU WANNA USE CUSTOM PLAYER MODEL MODS WITH GOOD COMPATABILITY.