Green Hill Zone
Of course, we begin with the zone that needs no introduction. Green Hill Zone is the first zone in the entire series, being level 1 of the very first game. Whether or not you think it's overused and annoying, there's no denying that it's one of the most iconic things in the Sonic series, period.
Just like its appearance in Sonic 1, Mania's rendition of Green Hill is very wide open, spacious, and simple, being a perfect place for any newcomers to come to grips with the controls and physics. Additionally, all but one* of the Badniks here are also present in the original Green Hill, how wildly appropriate. There are a lot of references to talk about, so let's not waste any more time. Let's-a-go, as they say.
* by one, i mean two. we'll get to that later.