I confess, I find it hard to identify a particular spell that is most useful in dueling. In my own humble opinion, I believe that the structure of dueling tournaments is missing one key component.

The use of potions should be permitted in dueling, and their incorporation, I think, would be the greatest asset a duelist might come across. Potions (including poisons and their antidotes) have myriad applications and can help to undermine the "traditional" dueling rhythm of stunning, shield charms, and disarming that has become so overdone. Incorporating poisons would add a dimension to the skill of the duelist, making them a more rounded sorcerer.

In conclusion, spells have their place, but it is potions that truly expand the sport.