I think the third game, despite the many people who have complained about one point or another, is actually pretty decent.
But there is a plot hole that most people haven't yet realized: Why didn't Diablo just destroy Imperius, Auriel and Itherael? And come to think of it, why didn't Tyrael join the nephalem in the final battle against the Prime Evil?
Diablo himself told the player that Auriel was captured by Rakanoth...but what I don't understand is why Auriel was captured in the first place - if the demons were strong enough to overpower her and so intent on crushing the angels' Hope forever, why didn't they just finish her off? They must have known that there was at least a chance that she and Itherael would be set free...