Whether you like Sonic in 2D or 3D boils down to personal preference, but Sonic's 2D track record is much more consistent. Most of the 2D games are objectively good, though. One consistent thing about the 2D games includes the existence of a special stage, with most of them needed to get a chaos emerald.
The stages get increasingly more difficult until you get all seven chaos emeralds. These emeralds will allow you to turn into Super Sonic and often unlock the best ending. Some of these special stages are iconic and a lot of fun. Quite a few are duplicated or redone in other games and can be better or worse than their originators.
8 Sonic 3: Blue Sphere
The Blue Sphere special stage in Sonic 3 is iconic but also incredibly difficult. You must explore the levels to find hidden giant golden rings to access them. Once accessed, you need to turn every blue sphere red or into a ring. It sounds easy, but it's not. The first few are simple enough, but the rest can be borderline impossible for you.
Sonic's controls are very weird in these stages. You continuously move forward, and pressing left or right will rotate you in that direction. Since the stages are quite fast, combined with restricted movement, these sections can simply be too much to handle. One mistake can also be it for you, as touching any red sphere will immediately end the stage. This is the kind of section you'll either love or give up on.
7 Sonic 3D Blast: Endless Runner
Sonic 3D Blast has three different special stages depending on the version played. The PC and Saturn versions use the famous half-pipe as the special stage, but the genesis version is more interesting. That version's stage plays like an early endless runner, with you needing to collect a certain amount of rings before the goal and avoid the spikes. It's a lot of fun and a major highlight of 3D Blast.
The way to access the stages, though, is a bit weird. Hidden in many of the levels are either Tails or Knuckles, and being near them with 50 or more rings lets you play the special stage. How are you supposed to know that? Why isn't it more convenient, like all the other special stages? This holds it down a little, but it's otherwise a great special stage.
6 Sonic 4 Episode 1: Remade Rotating Maze
If you tend to get dizzy easily, Sonic 4 Episode 1's special stages might not be for you. If not, then it's going to be a good time. The special stage is accessed the same way as Sonic 1, collecting 50 rings by the end goal and jumping into the ring. Sonic 1's special stage is used, but you control the stage instead of Sonic, which makes the sections a bit easier.
It does feel much better to control than the first game, but the later stages are a bit lacking. Many of the later levels are huge mazes, with you needing to get a lot of rings to open the pathway to the emerald. These stages can just be boring, but some are actual obstacle courses, and those are great.
5 Sonic 1: Original Rotating Maze
While the original Rotating Maze in Sonic 1 is a bit more clunky than the remade version, that ultimately makes it more fun and engaging. You control Sonic instead of the stage, making it much more tense, especially near the end. The stages always end with the chaos emerald surrounded by gems that take a little while to vanish when you're near them.
With the stage constantly rotating, you're just praying you can get to the emerald before you're driven back. The rotate button is also a nice addition that adds more complexity and decision-making. You must think whether it's worth it to change directions if the rotation isn't quite to your advantage. The levels are also far shorter than Sonic 4's special stages, and overall the original Rotating Maze is better.
4 Sonic CD: Destroy The UFOs
Sonic CD offers one of the more unique special stages. Collect 50 rings by the end and jump into the ring, like Sonic 1, to access it. Once in, the stage tasks you with destroying every UFO. The freedom of movement sets these stages apart from the other genesis era Sonic games. The Rotating Maze, Blue Sphere, and Half-Pipe all have restricted movement to some degree.
In Sonic CD's special stages, you have total free movement, making it much more fun to play. It's still challenging but also a joy to go through. It is worth noting that collecting all the time stones is not the only way to get the best ending. You can achieve a good future in every zone manually with the time travel mechanic, but the preferred method will be personal preference.
3 Sonic 2: Original Half-Pipe
The Half-Pipes are the most common special stages in the Sonic series for a reason. It's a lot of fun and doesn't overstay its welcome. Sonic 2 was the first to do it, and while it's fun, many of the future Half-Pipe stages are superior. Accessing them is much easier and less of a hassle than before.
You just have to get 50 rings when you pass a checkpoint and jump. It's that simple, and it's very hard to miss, unlike Sonic 1's method. The stages are a solid first attempt, but objects appear a bit too fast, and playing with Sonic and Tails is a nightmare due to Tails not being in sync with you. Playing with Sonic solo is recommended and more fun.
2 Sonic 4 Episode 2: Remade Half-Pipe
All of the flaws in the original Half-Pipe are fixed in Sonic 4 Episode 2's version. The controls are much smoother, it's easier to see the objects coming, stages are more dynamic, and Tails isn't an issue anymore. Above all, it's still really fast. Unfortunately, accessing them is through the Sonic 1 method. As such, you will need to make sure you don't fly past the gold ring at the end of a level.
The final few stages are challenging in the best way, and it feels incredibly satisfying when you get all the emeralds and can turn into Super Sonic. These stages are thoroughly enjoyable, and you may see them as one of the big highlights of this game.
1 Sonic Mania: Catch The Emerald
The best special stage in the Sonic series, by far, has to go to Mania. It plays similarly to Sonic CD's special stage, but the goal is much different. To access them, like Sonic 3, you must find giant golden rings in the levels. The goal in these stages is to catch up to the UFO holding the emerald and take it.
You have to go fast by getting blue or yellow spheres, while getting rings adds to your time limit. These stages have the biggest fun factor and best controls, but what really makes them the best overall is the main goal to go fast. There are no obstacle courses or restricted movement. Just focus on speed, and that's what Sonic is all about.