Eh. I really don't like the majority of 2D Mario. I find that Mario games are only somewhat enjoyable the first time though, and get even less interesting if you try to replay it. The good Sonic games are enjoyable the first time though, and get even better on replays.

My earliest gaming that I could really put time into (ignoring stuff like playing SMB3 for a bit at a neighbors' house, or PC games like "Gizmos and Gadgets") was the Mario Land games on Game Boy, and the 90's Sonic PC ports (Sonic CD and Sonic 3&K), and those are still the kind of level design that think is the best. A 2D level should have variety, every part should be memorable and unique, and it should want me to play it again.

I hate the normal 2D Mario level design style (SMB1, SMB2J, SMB3, SMW1, and NewSMB series) because so many 2D Mario levels feel like they take one obstacle, then use some procedural generation to repeat it any pad out the stage. Too many scroll purely in one direction, and lack interesting secrets or alternate paths.

Mario Land 1/2 and the Genesis Sonic game levels have landmarks and parts with obstacles or layouts that were totally unique or only repeated once, so you knew for sure where in the level you were. One of the first levels in SMW is just a bunch of spinning platforms in a row. If you were shown a short gameplay clip of that level, could you tell if that's the 3rd, 4th, 5th, or 6th spinning platform? Probably not, but there's only one place in Green Hill with a checkpoint between two s-pipes, so you'd know exactly were in the game that is if you saw it.

With SMB1, the repetition and one-way level design is understandable, due to technical limitations. But it stayed with Mario, and even in the New SMB games, many levels emulate the same, boring linear style.

Besides the Game Boy Mario games, Western SMB2/Doki Doki Panic and Yoshi's Island have good, interesting levels. The GBA and DS Sonic games' levels are poor because really fail at having good landmarks and the levels feel same-y.

In Mario Land 2 you could find secret exits that would lead to entirely new levels. Sonic levels have paths though the level that are more difficult to stay on, so you want to try again to see what you missed when the first time though. Sonic 1 may have levels like Marble Zone that don't really fit the idea of going fast, but they're still interesting and enjoyable if you look past that, far better than the average SMB3 or SMW level. Other 2D Mario games did have some exploration in bits, but it wasn't has good as Land 2 or Sonic.

Also, the ground controls for console 2D Mario kind of suck (the air controls are fine, though). SMB1's controls were good compared to other games of the time, but they've aged poorly and don't really fit the game. If you want to move at a decent speed, you have to hold B all the time for some reason, Mario's acceleration is too low, and I guess he greases his shoes considering how he decelerates. The Game Boy Mario games have much better, responsive controls than the "normal" 2D Mario games (but you still have to hold B, though). Sonic has the spindash, so he can accelerate fine, and his deceleration feels right.

I've wondered if my opinion of the Genesis 2D Sonic's was because I had played the games so much because I didn't have other options and I learned to accept any flaws. I wasn't a fan of spindash-less Sonic 1 at first, but when I tried hacking the game and ended up replaying parts of it a lot, I started to appropriate it more, so maybe repetition was a part of it? But later then Sonic Mania came out, and it was exactly what I thought I wanted, I enjoyed it as much I thought I would.