Ekalavya, a Nishada prince, learned archery by secretly watching Dronacharya teach the Pandavas and Kauravas. His skill was so exceptional that Dronacharya demanded his right thumb as gurudakshina to ensure Arjun would remain unparalleled. The other options are all archers from the Mahabharata era but were not considered superior to Arjun during his training period.