I have to agree with Mystic, Greenflower Zone Act 2 is just purely amazing. When I first downloaded SRB2, after playing Act 1 I was slightly disappointed, but right at the first instant of GFZ2, my view completely changed. Immediately I saw an enormous area with seemingly endless pillars which I wanted to explore, but at that point couldn't (because I was Sonic xD).
The level had many different paths you could take, and quite a few hidden secrets. In fact, I didn't even know about the super spring until quite some time later, when I accidentally stepped on it ^^".
I also love the placement of springs in that level, and how the camera angles alter to follow the character while moving on those springs. This is the defining level that I think of when I hear the name SRB2.
It's even fun to just run around on the water (near the waterfall) with supersonic ^^. I just ran around for 20 minutes once. Unfortunately, I had to keep restarting to get rings back xD. Also, often times I play this level slowly, exploring every inch, just because it has such wonderful level design, and the music is great too ^^.