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.
Join our Matrix community space! #sonic:pawb.social
Welcome to c/sonic on pawb.social, the largest Sonic the Hedgehog community on the fediverse!
Come and discuss the radical blue blur and his friends!
1. Be respectful
c/sonic encourages discussion, but all interactions must remain respectful. Harassment, trolling, drama, shipshaming, or any offensive/bigoted content (e.g., racism, sexism, homophobia, transphobia, ableism, xenophobia) are not allowed. If you encounter harassment, report it instead of engaging. Responding with similar behavior may result in moderation action. Constructive criticism is welcome, but if it’s not respectful, don’t post it.
2. No sexually explicit content
Pornographic or sexually explicit content is prohibited. Non-explicit NSFW content must be tagged appropriately. Moderators may remove or restrict content deemed too suggestive.
3. Stay on topic
All posts must be related to the Sonic franchise. This includes the games, comics, movies, TV series, fan content, etc.
4. Credit creators
Always credit original artists in the title and link to the source in the post body or comments. If the source cannot be found, explain your efforts. Respect creators’ repost policies. Do not claim others’ work as your own - this may result in a ban.
5. Spoilers
Use spoiler tags for content released within the past two weeks.
6. Reposts
Avoid reposting recent or commonly shared content. Moderators decide what counts as a repost.
7. No AI Content
AI-generated content is not allowed. We support human creators.
8. No Spam
Spam includes repetitive posts, copy-pasted comments, or low-effort content. Repeated violations may lead to a ban.
Links
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 grew up playing the 8bit Master System version of Sonic 1.

Might be fun to go with that first as it’s harder to go back to old stuff after playing newer.
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.
I'd recommend Sonic 2. Provides enough speed and platforming without sacrificing either one like Sonic 1.
For me it was Sonic 3 (& Knuckles) for Genesis. Sonic Mania is a close-second spiritual successor.
Sonic Advance
You can emulate it for free with mGBA and get the game from vimm.net.
I like sonic advance 2 also!
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.
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
Generations is probably the best way to dip your toes in with the 3D games
Sonic 2 imo. 1 is frustrating and then you can play 3+Knux after.