Twitaholic ranks the most influential tweeters by number of updates (top) or number of followers (bottom).
You may be surprised to find that the most frequent posters aren't necessarily the ones with the most followers, and vice versa.
For additional tools to identify influencers on various social media channels, check out the lists at Binkd or Ragan.
These sites can help you identify people you might want to follow for research purposes.