I am not a muffin person--they're usually leaden, tasteless, and huge--but I had a great muffin this weekend at the Silver Moon Bakery on B'way and 105th. Chocolate chip streudel, with a nice brown sugar streudel top and a fair portion of chocolate without being too chocolatey. Most important, it was actually not too dense.
I didn't see if they had any other flavors, but it's worth checking out if you're in the area. A little pricier than what's around ($2.25) but worth the freight.