Most surprising series has gotta be Bruins/Habs. Or maybe it's not surprising: maybe it's just enlightening. Just before the playoffs, I read a thing about the most common upset every year being a 6-seed beating a 3-seed. I thought the Coyotes were a better candidate for this than the Habs; this is because I am an idiot.
Looking at it now, the contrast between the Coyotes and the Habs couldn't be more interesting: in the West, you've got a team that slows everything down and tries to go to OT every game to salvage a point--in the East you've got an undersized team that seems somehow built to beat teams in the playoffs after somewhat underwhelming regular seasons. Just weird is all.
Or maybe there's always a 6-3 upset because the 3 seed is often a the best team of a weak division, piling up points against glorified major junior teams, but completely unprepared to play a good team 4 to 7 times in a row. (Checks Northeast division.) Naaaaah.