Duel reward card is one of rewards, including drop skills, stones, and jewels, for winning a duel. What card you can get is different by which legend duelist you win a victory over.
Staple and versatile cards that work well in any kinds of decks are on a tier list here. That means the best reward cards to build a deck with a specific theme will differ from the list. For example, you may want to have duels against Mako Tsunami, who mainly drops Water-attribute monsters and their support cards, when you plan to use a water deck.