Okay, my experience with wizards in fiction may not be as extensive as some of the other posters, but here goes.
I have seen people mention Gandalf, Dumbledore, Merlin, Raistlin Majere, and Robillard. These are all wizards I have known of, and all of them have decent arguments for being the best wizard in fiction.
Dumbledore was the most powerful wizard in his world, and only died because he was poisoned and had to be mercy-killed. Robillard took down an immortal lich wizard way more powerful than him while below decks on a sinking ship. Gandalf was a demi-god that killed another demi-god, died of his wounds, and then came back more powerful than he was before (a fact which causes me to laugh at anyone who thinks Dumbledore would be a match for him). Raislin Majere killed an entire pantheon. Merlin is such an iconic wizard that almost all of the greatest wizards in fiction have been based off of him or pay tribute to him in some way.
But how, how in the hell, has it been this long before anyone has mentioned Negi fucking Springfield? Does no one on the Escapist know who I am talking about? Anyone who has read up to the fight against Jack Rakan will know exactly what I'm talking about. Here is a kid who went from being a weak, fresh out of the academy, child wizard who could barely handle a dodgeball match between some obnoxious girls to being an incredibly competent warrior spellcaster capable of going toe-to-toe with someone who ripped a dragon deity in half with his bare hands. And he earned every drop of his power through hard work, training, intelligence, and innovative magical theory. He was able to ensoul a robot by kissing it really, really hard. (That's not really important, but I think it is too hilariously awesome not to mention.) And did I mention, this super-mage is only 11 years old?
I mean no disrespect to any of the other wizards mentioned here, I don't even know anything about Harry Dresden or Rincewind, and I've only heard a little of the accomplishments of Zedd or Ged. But I have mentioned the accomplishments of the wizards I do know, and I can say with confidence that at this point in his story, with his incredible speed, power, and combat prowess Negi Springfield would almost certainly (barring a surprise attack, maybe,) crush any of the wizards I mentioned before they had time to speak (or possibly even think) the first word of a spell (although I will admit getting Gandalf to stay dead may pose a bit of a problem for him).
The fact that it has been this long before Negi Springfield got so much as an honorable mention is a tragedy.