The best kokorec is in the Balikci pazari off of Istiklal Cd. If you are heading down to tunel square from Taksim, it will be on your right hand side near the Galatasaray lycee. The street has a great iron awning, and is always really crowded. There's about a billion guys selling stuffed mussels, fried mussels, calimari, and kokorec, and because of the volume, it's pretty fresh. As a bonus, this is one street over from Nevizade sk, where you can get great meze and raki.