The Japanese call it gobo, while the English call it greater burdock. This root of the Arctium lappa plant packs some major antioxidant punch. It has an ORAC value which trounces raw carrots (697), Russet potatoes (1,322), and even purple sweet potatoes (2,720).
The highest antioxidant root vegetables would be ginger, turmeric and other spices, but you can't eat those in vast quantity. Among the veggies which are not a spice, it's hard to beat the burdock root.
Research Support, Non-U.S. Gov't: Antioxidant Capacity and Polyphenol Content of Extracts from Crops Cultivated in Japan, and the Effect of Cultivation Environment. Food Science and Technology Research. 19(1):69. FSTR 2013