So I think we can all pretty much agree that, except for Remus Lupin (check out the clip below!), Dumbledore's choices of Defence Against The Dark Arts (DADA for short) teachers were all pretty horrible. They were all either dark wizards, hot headed amateurs or pink monsters that everyone wanted dead more then the actual villain (yes, we are looking at you, Umbridge) and with teachers constantly changing, it's to be expected that it would take Harry Potter seven years to defeat Voldemort. However, I have compiled a list of people people who, if their resumes had been sent to Dumbledore's desk, would have gotten the job in a blink of an eye (and with Professor Mcgnagall's approval, no less!)
Gandalf The Grey
The only wizard that could give Dumbledore a run for his money would most definitely be the great Gandalf of Middle-Earth. He had somehow managed to help and teach a small group of Hobbits how to survive against one of the greatest villains ever created. If you managed to survive his extreme tuition, you could survive anything.
Sam and Dean Winchester
Ah, you've got to love the Winchester brothers! Apart from being a good source of entertainment, with their childish bickering and and brotherly teasing, the Winchesters would teach you how to travel to Hell and back if it means saving the life of someone you cared about. With their dad's trusty journal and over a decades worth of experience, they'd teach you how to deal with any unruly school ghosts (Peeves better stand clear) or strange creatures the might crawl out of the Forbidden Forest.
If Giles was able to train the amazing vampire slayer, Buffy, he's just the man Hogwarts students need to teach them DADA. Apart from having a limitless knowledge of all things dark and magical he could provide them with just right enough defensive training to get any student out of a pinch, especially if they had somehow been disarmed and are lacking a wand.
With almost 2000 years of experience up his sleeve and a Tardis always at the ready for a convenient field trip to some monster infested planet, the Doctor would be a perfect candidate for the job, if he ever got over the shock of seeing an actual ghost, that is. Some explaining might be needed before he'd wrap his head around that one.
A walking encyclopaedia, Annabeth Chase from the Percy Jackson series knows all about mythological monsters and, let's be frank, those are some of the most gruesome monsters out there. She doesn't need a wand or spells to save the day, just give her her trusty baseball cap (which, come to think of it, might just be a modified Deathly Hallow) and she'll be having any dark wizard or monster begging for mercy.
So that's the line up! Dumbledore has 6 amazing teachers to choose from and each one has one single message to send to all monsters and dark wizards out there - you will regret the day you ever crossed paths with them or any of their students!