Nasser David Khalili

Art collection: $900 million

Iranian-born academic and former art dealer has one of the largest private collections of Islamic art with over 20,000 items; includes rare paintings, textiles and manuscripts (like the one pictured). Bought his first piece, a 19 century Qajar lacquer pen box, as a teenager. Also collects Japanese Art from the Meiji period, Indian and Swedish textiles, Spanish metal works. Nicknamed the secret sultan, left Iran before the 1979 revolution and now lives in London.