28 Nov 2010 at 00:56 - 18
I realize you're asking about the game right now, but here's my historical perspective, since I played each campaign as it was released:
Prophecies is long and slow if you plan to actually play through the whole thing. But I guess it's pretty easy considering, especially since you can skip so much of it. When I first played through it though, it could be a nightmare in parts. Having to find a monk or rely on Alesia was often a bigger challenge than the missions themselves.
Factions is likely the fastest, some of the missions take only minutes if not seconds, like killing Shiro. Some healers had trouble with the Afflicted back in the day, but that isn't really as much of an issue anymore.
I remember having some issues with Nightfall when it came out, but that was because I was working a lot that first week and was essentially "left behind" by my guild and forced to pug it up to the Realm of Torment. Still, with the heroes, skill and gameplay improvements, and my own increased skill by that point, I felt it was the easiest overall.