I played through Sonic 2 as Knuckles through the Switch version. I'm impressed that Sega managed to add a completely new playable character to a (at the time) two-year old game, complete with all of his abilities, when he was never intended for that game (and didn't exist when it was made).
Not only that, but they even kept the wall-climbing, which really opens up the options for exploring the levels even though that was never intended. I get that Sonic 3 and Sonic & Knuckles were intended to be one game that was split apart for time constraints, but the Sonic 2 bonus seemed like it went above and beyond.
Also, I have no idea why I've bought Sonic 2 at least five times. (in order, it's been Sega Smash Pack 2 for PC, Wii Virtual Console, Sega Genesis Collection for Steam, Sonic Mega Collection for Gamecube, the Switch version - ironically, I've never owned a Sega Genesis or had a Genesis version of the game) I guess Sonic Mania got me wanting to play one of the classic Sonic games on the Switch.
A lot of people force themselves to sit through a movie, book or in this case a video game just because it's a "classic", we all have our own classics after all.... But yeah anyways, OOT is really good :v