Dumbledore says that there are only three wizards in Hogwarts powerful enough to pull off what happened in chapter 74, and names Snape among them, but my impression from the original novels wasn't that Snape was exceptionally powerful among the professors of Hogwarts. He was definitely extremely formidable compared to the average wizard (one thing that I thought added verisimilitude to the books was that it was frequently shown that ordinary adult wizards frequently aren't very competent at magic aside from the few things they use regularly; think how many adults in real life don't understand or know how to apply much of the material that you're supposed to have to learn in order to get through high school,) but it seemed that many of the professors of Hogwarts were on the same level. He and McGonagall duelled in the last book, and it seemed like McGonagall had the upper hand. Plus, Flitwick was a former duelling champion, so I imagine he's pretty powerful as well. Snape is the most knowledgeable person in Hogwarts about the dark arts after Quirrelmort (who he probably learned at least a bit from,) but if I had to guess the most powerful professor after Quirrel, it would have been one of those two.
On a side note, I have to wonder if Quirrelmort has any idea about Snape's betrayal of him? What he's doing might just be messing around and showing up one of his former servants, or it might be part of a protracted vengeace.
it seemed like McGonagall had the upper hand.
I don't know -- even in canon, Snape didn't want to win that fight.
But this does leave me slightly confused about his apparent stunning just now.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: