When I think of Sonic and why I love it, it all boils down to Sonic Adventure 1! Sure it has it's flaws, many, MANY flaws! But there is something cozy about it's atmosphere. The little hub areas between levels are fun to explore and add a level of exploration the level based games just lack. I love the cutscenes and the story, it's the first time a story REALLY felt present within a Sonic game and I was fully very invested in it - Like with all the lore around the ancients and the Master Emerald. SA2 just wasn't the same, its pure level based gameplay was a pain, especially having to dig through hidden upgrades in levels and some levels like tails/Robotniks just werent fun.
Beyond that, I enjoy S1-3&K but found level design for CD & Mania to frustrating. A lot of probelms with Sonic in general are through the hit detection - I often end up spamming bosses when I'm immune after being hit or fuming whenever the game thinks I get squished.
I think that's a great way to sum up my feelings towards Sonic Adventure as well. It's certainly not my favorite game but there's something really charming, cozy and nostalgic about it. I think it still looks great, has some interesting level ideas and has a FANTASTIC soundtrack. SA1 is one of few 3D Sonic titles I enjoy.
I'm not a big Sonic fan. Outside of the original trilogy and S&K, I always found it hard to get into his games. SA1 is the bright spot for 3D Sonics for me, and one that really impressed me when it came out, singlehandedly made me want a Dreamcast (which I sadly never got) but down the line I got to play the Sonic Adventure games on GameCube. But even SA2 didn't quite do it for me. The only other Sonic 3D game I liked was Generations. And I played all of them except Frontiers. Why I keep trying them, I have no idea. I think there's a part of me that believes I like Sonic games but then I sit and play them I can't get into them.
TL;DR: 2D Sonic is better, and just works better for the type of game.