Shares is the single most valuable metric you can track. By looking at what your audience is sharing, you get two key insights: 1) who they are (those who spread the word) and 2) what content they are interested in.
In Twitter, only a small group of users will take the risk of retweeting your tweets. They won’t share a tweet at least it contributes to their social presence in some way.
You can get the most retweeted tweets for a given topic (that you can set here) by running this script.