They all have their +/-'s.
The original wild wargs are very good, you can't really say there is anything wrong with them. The flat backs I assume were just because they were taken from warg riders, but it is really easy to stick some GS on and sculpt it to be fur like. I like that they are covered in fur which gives them a very standard look. They can be painted to a high detail or to an amateur standard and still look very good, so it is dependent on what you are looking for.
The fell wargs are obviously better in the pose department and look very realistic in this sense. As others have said, I am not such a fan of the transition from skin to fur, though there are hounds where this is exactly the case, so I wouldn't call it unrealistic, just uncommon because it looks so unsightly to many people.
These were made to be fell wargs and nothing else in my opinion. It is reflected throughout the whole sculpt. This means that you really need to paint them as such and are more restricted to highlighting the same areas (ribs, flanks, mane). They are excellent models, so this isn't really a problem, but it does mean that you should pick out all these details carefully or the model will not look as good, so it will take longer to paint.
The wolves I like but wouldn't get more than one pack of. I think the eyes are too light which doesn't fit them so you would need to be thoughtful as to how you painted them as they are much more significant and visible than the wargs. Anatomy wise I think they are very good, they have the best tails of the lot.
I don't like the sculpt work though. To me it appears lazy in the mid section with not enough detail that can easily be made out. The transition from fur ti skin I think is a little bare/stark in some places which to me doesn't really look wolf like.
If you would have been more specific as what you wanted, I wouldn't have had to write all that
Assuming you want to use these as wild wargs, I would rank:
#1 Wild Wargs, #2 Fell Wargs, #3 Wolves. Leave the fell wargs for more significant models to make them stand out. Wild Wargs are easiest to paint to a tabletop or better standard. Given some flock on their base they would look much better as well.