There's a common misconception that old Sonic games were all about speed. That was how they were marketed, for sure, and some of the more recent games may paint a picture of that, that wasn't how it was. The old Sonic games were more about momentum, with speed just being one part of that puzzle. This focus is illustrated in the infamous Green Hill Zone, where Sonic's signature power is made use of. The terrain isn't flat, like it would be in a Mario level, even stretches of functionally straight ground weave slightly up and down to keep the terrain varied. Arguably the staple element of Green Hill Zone are the loops, which are themselves an expert use of the games physics system. You can't just run up them at will (In the old games at least), you need to actually build up enough speed to reach the top, which will then translate into an increase of speed as you descend the loop. The physics were the core component of the game design, and they worked in tandem with the level design to create a satisfying sense of speed.
While the physics always remained consistent, the level design varied between the 4 classic games (1, 2, CD, and 3&K. Mania is more or less the same philosophy as 3&K, so I won't talk about it.) Aside from Green Hill Zone, Sonic 1 had a very mixed set of levels. While there were some other good ones, like Spring Yard and Star Light Zone, both levels only really focused on one half of the Sonic dichotomy (Momentum for speed in the case of Star Light, and momentum for exploration in Spring Yard.) The rest of the levels were just generic platforming levels, and weren't very fun as a consequence.
Sonic 2 has a much more consistent set of levels, but it comes at the expense of the exploration game. The levels are all super linear and speed focused, which isn't entirely a bad thing as the game has some of the series best levels (Chemical Plant and Casino Night), but it also means no level ever really reaches the highs of Green Hill Zone in the first game. On the flipside, Sonic CD has the opposite problem. The levels are super dense and multilayered, but in doing so the levels become super convoluted and slow. The only goal of some of them seems to be to waste your time, which is never fun.
Sonic 3 & Knuckles was the best Sonic game because it decided, ah **** it, and just combined the level philosophy of all the previous games. It's got the speed of Sonic 2, the exploration of Sonic CD, and the satisfying design of Sonic 1. Basically every level in this game is great (with maybe one excpection), and the game as a whole is just a lot of fun. Mania is basically the same shebang, minus the annoying ghosts. What really makes all these levels great is how they use momentum to achieve both speed and platforming, often in tandem, and create satisfying stages.
I'm going to reset now. The hope is that the magnitude of de-stressing this gives me is greater than the magnitude of the stress it causes you, so that the total amount of stress in the world goes down.