Although not as well known as some of London's other popular outdoor markets, Broadway Market is arguably one of the best in the city. From haggis sandwiches to vietnamese coffee, the endless, diverse food options make this a great culinary destination. If the weather is nice, make sure to bring your four-legged friend! LINK.
Runner Up(s): Jemaa el-Fnaa, Marrakech; Christmas Market at Rathausmarkt, Hamburg