1. Dumbledore - Though with Harry's mother's blood to protect him Voldemort is pratically tied, Dumbledore found out about the Horcruxes and already thinks he knows what all of them are. He is, no doubt about it, the smartest wizard/witch
2. Voldemort - who else?
3. Snape - By HBP Harry is very strong, yet he couldn't even harm Snape. Snape fooled Dumbledore.
4. Quirrell - He fooled Dumbledore, nearly stole the Sorcerer's Stone from Hogwarts and would have stole it from Gringott's had Hagrid not took it out. Now, it's supposed to be impossible for someone to steal something from Gringott's and the stone was in a top security vault. Yet he would have stole it had it been there. At the Quidditch match Quirrell nearly had Harry off his broom, even with Snape trying to keep Harry on the broom, which suggests that Quirrell is stronger than Snape. However, I will say by HPB Snape is stronger than Quirrell.
5. Bellatrix Lestrange - She easily beat OOTP Harry. She also beat Sirius and Kingsley. Before fighting her Kingsley had beat 2 Death Eaters at the same time.
People who deserve honorable mentions: Flitwick, Rufus, McGonagall, Sirius, Lupin, Kingsle, Amelia Bones and Barty Crouch Jr.