Everyone always brings up 'Harry Potter' for this, and yeah, those duels have their moments, especially in the later books with more serious stakes. But if we're talking about duels that are built into the fabric of the magic system itself, where the mechanics of the fight are the point, you gotta look at the progression fantasy genre.
Take 'Mother of Learning'. The whole story is structured around a time loop, and the protagonist uses it to dissect magical combat down to a science. The descriptions of spell matrices, mana control, and counter-spells are incredibly detailed. It's less about dramatic flair and more about tactical problem-solving, which I find way more satisfying when it comes to the idea of a 'powerful' duel.
Then there's 'The Combat Codes' by Alexander Darwin. It's not wizards in the traditional sense, but the magic-adjacent martial arts system leads to confrontations that feel like magical duels in everything but name. The precision and strategy involved create a similar kind of tension.
For pure, unadulterous spectacle though, the final battle in 'The Fifth Season' involves a conflict that reshapes continents. It's wizard-adjacent too, but the scale of power on display is almost incomprehensible.