Hands down, beyoglu is the best. Try going down from Tunel square to Karakoy or off the side streets of Istiklal for some really vivid stuff. Cihangir's graffitti is slightly stuffier, but has the advantage of often being written in English and being a lot more cerebral. Right now the whole city is covered in yellow fists and black stencils of Amy Winehouse from her aborted concert over the summer, but there is plenty of original stuff, although you'll probably need a turk to explain what it means or what it refers to.
