2. Gellert Grindelwald
His duel with Dumbledore went down in history as the greatest duel ever fought between two wizards. He probably used more than the 'Expelliarmus' spell and rightly so because usually, if someone is trying to kill you, disarming them isn't the first spell that comes to mind (apart from when you're Harry). He is referred to as one of the most powerful Dark Wizards of all time and he deserves second place, but sadly without the use of horcruxes and despite possessing The Elder Wand, he couldn't manage to defeat the undisputed winner...