I am surprised that people get lost in this one, going through it again I feel like this and S3 are some of the more linear ones. The level design is the most complex but usually with only a few main paths. The levels aren't hugely open like they can be in Sonic 1 or CD.
I will admit to not loving Sandopolis. Lots of weird gimmicks in both stages that slow it down. But really not that bad compared to some of the S3 levels. I'm not the biggest fan of some of the bosses either, especially the first one at the end of Death Egg haha, takes as long as it takes to get through the level to beat that thing.
Also never been a huge fan of Sky Sanctuary, but I guess most like it apparently? What other people are suggesting. Too many pits (I don't like Sonic 2's Wing Fortress either) and the backgrounds are a bit plain. It kind of feels like a stripped down Metallic Madness good future from CD.
It's a noticeable step down from how good the Lava Reef levels looked to me, I always wish the temple stage with Knuckles was more of a real level. Lava Reef is so awesome, one of my favorites in all the games. For whatever reason 16bit caves always ruled. Lava Reef, Yoshi's Island cave levels, Super Metroid, etc, caves always ruled haha.
The additional powerups for Sonic were a very cool addition. Just kind of funny to get the bubble in S&K when there's barely any water, more indication that the two were probably meant to be one big package originally. I prefer Sonic's look in these compared to Sonic 2, but still like the stubbier Sonic in 1 and CD the most.
Death Egg is one of my favorites and a standout, because it was such a tease at the end of 2. You get there, it's this massive Sonic Death Star... and it's just the final boss fight haha. Even as a kid I wished it were a full level and so that blew my mind with S&K when I got to it.
Other silly beans... I like the bigger blockier look of the powerups in 1, 2, and CD, and I've always disliked how the animal trap things at the end of the levels in S3&K only release like 8 animals, compared to the dozens that Sonic 1 does haha. Just silly things I've always thought about over the years with them.
I like how S3&K handled the bonus stages though, hidden large rings in the levels. In Sonic 2 it's always tedious having to hold onto a bunch of rings and find checkpoints. Bonus stage wise I like S3&K's a lot and they've always seemed the most doable to me. Sonic 1's get really evil near the end and there's always 1 or 2 stages in Sonic 2 that trip me up. That said, S3&K seem the most abusable with the bonus stages, as others have said it's really easy to have all the Emeralds really early in the game if you know a lot of the hidden spots.
You know... I just realized, I feel like S3&K have the most "pits" in the classic games. I'm having a hard time thinking of any in CD right now. I know there's some in 2 off and on, and maybe the original here and there. It's always been up in the air between S&K and CD for me, but I usually do lean towards CD being my favorite in the end. I feel like there's not a single level that bugs me and the level design is consistently great, and the game is just downright gorgeous. Kind of a shame we didn't get more ~32bit Sonic's during the Saturn era that still had sprites, so... more Sonic games that looked like Megaman X4-X6 or something. I know there's Knuckles Chaotix, but it's always looked pretty weird to me. Never played much.
For final stages I still think Sonic 1 takes the cake. We all grew up with those insane water stages and Sonic 1 took it to the next level with its final stage. It's brutal, challenging, and epic. The water stages are tough in Sonic 1, but that's why I love them heh. That and the looks.
I went on a Mario binge last year and was thinking of revisiting Sonic... this may be the beginning of another binge.
