Gilderoy Lockhart

And they let me keep my straightjacket!

Lockhart himself was a fake, but his books (which all the students had to buy) weren't. Those accurately described the exploits of genuine experts that actually did DADA stuff. Lockhart interviewed them and wiped their memories. So while Lockhart's classes were a waste of time, anyone could just read his works and get a solid DADA education. The students weren't actually left out to dry. Honestly, it was almost certainly a better year than Umbridge.

I want to go a step further and say that Dumbledore hiring Lockhart to expose him was brilliant. If Dumbledore had proof that Lockhart was wiping people's memories, he'd have him arrested and sent to Azkaban. Since that didn't happen, Dumbledore doesn't have proof, only suspicions. To expose Lockhart, he'd have to go searching all over the country for the people Lockhart wiped. Even if he found them, Lockhart's memory charms can't be easily undone, except by himself. And even after all that, there would need to be a trial, where Lockhart's looks and money might get him off easy.

On the other hand, put him at Hogwarts and make him teach class for a year, and it instantly becomes obvious he's a fraud. You have hundreds of students as witnesses, even paper evidence in the form of the useless homework Lockhart gave. I'm fully certain that if Lockhart hadn't lost his memory, Dumbledore would have used the overwhelming evidence to convict Lockhart, and force him unwipe his victim's memories.

Ready for my next book signing at flourish and blotts!!!! For the re-release of my last book "who am I?"