To determine best use of Twitter, much like in the previous category, I did not focus on the numbers.
The most important -- and inherently highly subjective -- thing is who is conveying useful information, concisely, accurately, eloquently, and in a timely way.
In talking about green business this can be doubly difficult sometimes. Thankfully for all of us, Adam Werbach does a thoroughly excellent job at it. You need to follow him.