play the original sonic the hedgehog. if you enjoy that, i suggest also playing these other 2d sonics:

• sonic cd
  • introduces the spin dash.
  • play the japanese version. they absolutely butchered the american release. sega of america decided that slapping house music is too much for innocent, christian, american children to handle, so they replaced most of it with a bunch of other shit that's thematically unfitting, but left the 'past' versions of the tracks alone because they couldn't figure out how to replace the non-red book pcm audio.
  • this game is easy to beat, but hard to find all the secrets in.
• sonic 2
  • refines the spin dash
  • has tails
• sonic 3 and/or knuckles
  • yes there's actually three combinations. they split the game in half so they could rush it out as 'sonic the hedgehog 3'. then they released the other half of the game as 'sonic & knuckles'. if you stuck sonic 3 onto sonic & knuckles, you'd get to play the whole game as one, so basically it was dlc for the prehistoric era of gaming.
  • very long, stages are quite complex
• sonic mania
  • 2017 game, homage to the old sonics
  • mixes old and new stages

don't play knuckles & chaotix, it sucks. also don't play sonic 4, it's a shitty cash grab.

i also recall sonic advanced being not terrible, which is high praise for a sonic game, really.