We wanted to know just that and wanted it in an easy format to browse.  Twitterholic was the first to come out with this idea but their data seemed to be always wrong and also very buggy.  There is also Twitdir which is pretty good data wise but we didn't like having to view only 5 entries at a time. (Twitdir also provides a good twitter user search capability)

So, wanting a better solution, we built a top 150 list for you, the readers of Twittown.  It is called Twit Rank .  

You can sort by top friends, followers and updaters and for each you can see the top 150...