The original for Genesis is a great place to start. It's not elaborate, it's challenging and fun, and has the basic tropes of the series.

Full disclosure: I've never played any of the 3D Sonic games.

I cut my hedgehog teeth on Sonic 2, which came with the Sega Genesis if I recall correctly. I never really played Sonic 1, but it was basically the same game engine, except they didn't introduce Tails the squirrel until Sonic 2.

For me it was Sonic 3 (& Knuckles) for Genesis. Sonic Mania is a close-second spiritual successor.

I'd recommend Sonic 2. Provides enough speed and platforming without sacrificing either one like Sonic 1.

I'd say play the first level or two of Sonic 1 and then (if you aren't feeling it) switch to Sonic 2.

This is assuming you're looking for a 2D platformer. If you're looking for a modern game with a mix of 2D and 3D, Sonic Generations is good.

Sonic 2 imo. 1 is frustrating and then you can play 3+Knux after.

I personally really like The Murder of Sonic the Hedgehog.. Sounds like a fanfic; it's an official game. Sonic does not really get murdered (you're doing a murder mystery game on a train).

But it also is not a typical Sonic game. Might be why I like it because the main games are not my cup of tea but I do like the characters. But Murder of Sonic is a fun visual novel. And it's free!

Sonic X Shadow Generations would probably be a good starting point, unless you wanted to start with the old 2d games

Sonic Advance

You can emulate it for free with mGBA and get the game from vimm.net.