While I'm more of a fan of Sonic 3 & Knuckles, I'd recommend Sonic 2 to anyone who hasn't played it, even if they just play the first zone. It's definitely speedier than Sonic 1, with less exploration than 1, CD, and 3&K, but it is still really good with it's level design and emphasizes the "not where you jump, when you jump" kind of mentality explained in stuff like Sonic Spitball (I'd recommend watching that if you're interested).