Sonic started off as a series that was conceived as a marketing gimmick from the core- it was Mario, but FASTER and COOLER and RADICOOL. Everyone knows this, but more to the point, it ended up accidentally (or maybe intentionally) structuring a core element in games that was practically unheard of 'til then - physics. Being able to move at breakneck speeds wasn't impressive in of itself - it was that you had an organic sense of movement, like a pinball going through a machine (pretty sure that was their basis for the physics anyway, which is probably why we ended up with Sonic Spinball). First pic related - it might seem like hyper 'tism, but that shit matters.
This "physics" element extended to the rest of the original game - from the moving and dropping of blocks in Marble Zone, to the obvious springs and crushing blocks of Spring Yard Zone, to the floaty water physics of Labyrinth Zone, to the see-saws of Starlight Zone. It almost felt like a demo of what the Sega Genesis could do, with an actual game built ontop of it. It was original, and really made Sonic stand out as a series of games, separated from the aesthetic and marketing.
Fast forward to Sonic 2, Sonic CD, and Sonic 3 (& nipples), and you'll see where they went from "loose tech demo" to "polished, fun series of games" built around that core idea of a momentum/physics tech demo. The problem is, they still had inherited flaws from the core game - the tech itself was still holding back the potential for some actually fun shit. From small screen sizes, to limited level sizes, and so on - to the point that the only game that's universally praised in the series required two fucking cartridges and "lock on technology" to work.
There's already tons of stuff written about why the "classic" games are best in general, but I will say that even that early on, Sonic has never been a "solid" series. Bits and pieces from each game are strong, while other aspects fall short. Sonic 1/CD had probably the best use of your (very limited) abilities, Sonic 2 had the ideal aesthetic (surreal with a touch of modern), and Sonic 3 had the best pacing. Altogether, they're great, but none truly "got it"- which is probably why Mania is one of the best games in the series- it's an amalgamation of all those elements.
Anyway, fast forward to 3D, and you'll immediately see what's wrong. They practically threw away the concept of physics, and put all that on the backburner for what effectively amounts to "lol you go fast and are 2COOL4SKOOL". There was no real direction or "core" to the game, so they feel like hollow experiences with varying degrees of fluff. The increasing focus on story, increased amount of railroading… again, tons of other autistic essays have covered this topic, but it all stems from them moving away from the core element of what made Sonic fucking good.
We could talk endlessly about the other "fluff" elements like the story, aesthetic, sound design, or whatever- but it all comes down to that simple core element of "momentum". Once that was thrown out, the games started to become a lot shittier. Even better was second pic related. The turbo autism community that wasn't invested in the games for what made the games good absolutely ruined almost all potential for fan input affecting the games positively - for every Sonic Robo Blast 2 or Sonic Mania, we'll have to put up with Sonic 06 or Sonic Boom.