Oh, good grief, where to start. I mean, I think for me it’s less about the games than it is about the world and the characters. The Sonic franchise is infamous for the large number of characters but there are so many that are well designed and have interesting personalities it’s easy for one of them to hook you in. I think it’s part of the reason why the Sonic fandom has so many women in it actually. The world has interesting lore, level design and music and what makes up the Sonic “aesthetic” is very interesting and easy to be sucked into. There is a ton of Sonic media to sink your teeth into, 5 TV shows, the OVA, the live action movie, the Archie comic which has nearly 500 issues, the Fleetway comic, the currently running IDW comic. Different genres of games, 2D Sonic, 3D Sonic, racing titles, a few interesting spin-offs.
As for the games themselves, well… alot of them are average or mediocre. Not really many that excel. Sonic 2 and 3 & Knuckles are the classic games, but even then they have aged a bit. I’d actually recommend Sonic Mania if you havn’t played that, it effectively keeps what makes 2D Sonic great but also trims some of the fat and old school jank.
I don’t know if that helps in anyway. I’d say Sonic Mania is a good start for 2D Sonic and Sonic Generations is a good start for 3D Sonic, both are on sale on Steam for £3.74 right now, and can be found cheaply elsewhere.