The Subway Station at Porte de Clignancourt
We went up to the flea market at Porte de Clignacourt and such a seething mass of humanity I have never seen. Once you get through the typical junky fare, the tourista stuff and the clothing, you do hit some cool albeit expensive antique shops. Though we did find some cool, inexpensive costume jewelry at one store in that area and had to buy one, well, maybe two bracelets.
Word of warning, buy your metro tickets before you reach Clignancourt.