An absolute 100% run isn't possible lest you edit the files to allow faction cross-over, since there's one quest per faction that can only be completed following that faction (at least, this is the case to the best of my understanding). But this sort of cross-over is pretty minimal, and you can still complete about 97% of the game's content in a single playthrough otherwise.
Is there a class with more possible quest than others? Or all classes have same % of quest possible?
What is (in your opinion) best class to play, I think that maybe some "class quest" are more interesting (or funny?) than others no?