From what I could find on Google, the "top tweet" designation means one with a lot of interaction & ReTweets, and some kind of "secret sauce" in the Twitter algorithm. We'll there's got to be a lot of "secret sauce" because there hasn't been any of the other so far.
It's also allegedly results in more followers because the tweet is featured. Will have to see about that. I'll keep you posted.
And of course, if you want to follow me, I'm @CherieTweetsToo, presuming that you don't already. ;)