Having lived in London for many years and adoring markets, I can safely say I've spent time at many. Spitalfields used to be my favorite a few years ago, but with all the gentrification in the area, I feel it's lost its charm. Yes, Portobello is a classic and well worth a visit and Borough has plenty of food—but if you're looking for the quintessential market experience, I have to say it's Broadway Market in London Fields, hands down.
It's in the coolest part of town, not too sprawling, and lined with awesome restaurants and coffee shops that sell foods like vegan and Bulgarian. There's a legendary arty bookshop, and you'll find cool stalls peddling vintage clothes, gourmet treats, and work by new designers. What's more, there's a big park (London Fields) to have a picnic with your freshly bought yummies amid great street art and people watching. This is a real market for creative, edgy types—a far cry from the type that has overrun Camden with Chinese Rolex knockoffs, incense, and marijuana paraphernalia. Enjoy!