I thought I'd try to bring this convo back to life.....
I'm gonna run through common neutral stages first:
Smashville: Flat ground, a single moving platform that can help sonic when looking to combo from below. I find myself trying to throw my opponent up onto the platform to catch them with an aerial after they land. The stage is great for trying to knock certain character underneath. All and all, a wonderful neutral for Sonic.
Yoshi's Island: Kind of like smashville... minus the flat ground and moving platform. I find that this stage can be frustrating at times when I try to execute aerials near the ground. Sonic players should really familiarize themselves with how their spin attacks are effected by the uneven terrain. I'm gonna go into training mode tomorrow and really tinker around with this one.
Battlefield: Another neutral stage that I am learning to love more and more. I've decided that platforms are Sonic's friend. This stage's two lower platforms are particularly useful for chasing your opponent.
Lylat Cruise: When I was a newbie with Sonic, I got stuck under this stage A LOT. I later learned that many other players with all other characters also shared my frustration. I learned to see this stage as battlefield with three perfect platforms. Sonic's speed really allows him to take control of this stage.
I just wanted to revive this topic as I mentioned at the start. If anyone has any comments/advice on other non-neutral stages I'd love to hear it.
Green Hill Zone!! 
