I don't know honestly, it's hard to pick the most specific 2D Sonic game whether it's Sonic 2, or Sonic 3. But, to me. Sonic 2 is my favorite 2D Sonic Game. It's Bosses are okay, they are sometimes harder. Most of the zones are fast-paced, and some are platforming. Its Special Stages might be worse than you thought from Sonic 1's Special Stages.
Sonic 4: Episode 2 is my second favorite 2D Sonic Game, now. I know that Sonic Fans might think that the Sonic 4 saga, had a horrible start. Leading Episode 1, to be the worst in the Sonic 4 saga. Except, Episode 2... Now, It does bring in content that doesn't repeat the original zones from the other 2D Sonic Games. As it has creativity, also. Its Special Stages are a copy of Sonic 2's Special Stages, except. Harder to go through, which I hate the most about Episode 2. Luckily, Episode 2 is technically an upgrade to its previous episode.
Sonic Mania is my third favorite 2D Sonic Game, it's because there reusing some zones from the other 2D Sonic Games. It's like they're reusing Green Hill Zone, but they're twisting it up, but it's still being reused in their next game. And I'm talking about Sonic X Shadow Generations. But getting straightforward, it does reuse some zones from the original 2D Sonic Games. But, it gives out brand new zones as well. Studiopolis, Press Garden, and Titanic Monarch. All giving their flair and uniqueness. Its Special Stages are fantastic as well, it's like Sonic CD's Special Stages, but you race for the UFO and get the Chaos Emerald. It's also really challenging (sometimes) but it's worth the try, at least...