I have always considered Dumbledore, Voldemort, Snape, and McGonagall the most powerful magic duelers in the books because they were (I think) theythe only ones who used transfiguration and other complex magic within their duels.
Dumbledore and Voldemort used it in the battle at the ministry in Book 5 (animating the statues, fire rope turned into snake, water sphere, etc.). Snape and McGonagall dueledused it in their duel at Hogwarts in Book 7 (torch to ring of fire, fire to snake, snake to daggers, animating armor, etc.).
Does anyone recall any other instances of wizards and witches dueling with magic other than curses, jinxes, and protection charms?