As a warning, if this devolves into a cat-fight, I'll probably request for this thread to be locked. This isn't about which zone is better, but rather why you're inclined to pick that particular zone. If you cannot grasp the fact that everything that will be said in this thread is subjective then you probably need a few more years to mature.
I guess I have no choice but to start us off. I've picked Emerald Hill because it was the first zone I ever experienced in a sonic game. Like I said above, this is an age gap issue. I wasn't old enough to know of the greater 'game industry' and knew of Sonic only through AoStH. At this time Sega was bundling Sonic 2 with the Genesis and I guess that's how it happened. Emerald Hill was fast paced, spin dashing and rolling through giant loops with so many secret paths. Sometimes I'd reset the game just to see which ways went where. The music was extremely catchy and is probably the reason why I've come to respect the bass as a musical instrument.
Since the day I played Sonic 2, I always wondered how awesome Sonic 1 must have been, but a few years later when I picked the game out at a thrift store, I had no idea I was going to be let down so much. The game seemed slow, less vibrant, and didn't have the fucking spindash. But on the zone in question, Green Hill felt amazingly bland and I still remember commenting to a friend how the "colors weren't as cool as Emerald Hill's." I never found the zone's music to be enjoyable either and still do not to this day. Ok, so obviously back then I was a kid and hadn't fully grasped the concept of technology and how it progresses over time, but whether I like it or not, this was a definite factor in why I prefer Emerald Hill over Green Hill. Ultimately the zone was a let down and when I finally broke free of its shackles and was thrown into Marble Zone, I found myself enjoying the game a bit more. For some reason, Marble Zone made more of an impression on me than the very first zone of the very first Sonic game.