Having only two litters I can tell you what I think. Angulations, easy, fronts not so easy. First litter with a sire that had an good front and my dame with a pleasing front, that was not good. Now in my last litter with same dame, but with another sire I have got very good fronts on most of my puppies. Another thing that is difficult is hanging eyes, it seem´s hard to change.Another thing is that the tail is a little velved up from the mittle and out. I dont know why but it seem´s to pop up in every litter. Must be something who is very strong in the genes. Hope that you understand everything I write here, because my English is not very good.
I think tail carriage is very difficult to improve. Fronts definitely are hard to improve especially since there are few dogs around with really good fronts. From my experience rear angulations and heads are easier to change, though it would depend on how fixed the type is (specially with the head) in the lines you are working with.
Alenka