Choosing just one is hard and it really depends on what role they fill. But a critical trait for anyone I work with: high ethical standards.
Naval articulates why this is so important in this 3 minute podcast.
Based on this quote of Warren Buffet:
"Somebody once said that in looking for people to hire, you look for three qualities: integrity, intelligence, and energy. And if you don't have the first, the other two will kill you."