This is a really hard poll for me! I'm going to vote on the faction I think is more cool from what I've seen in the Gwent stand-alone game. I like them all so far! But the one I found most interesting was the Monster faction with all the cool Eredin powers! In the built-in minigame, however, I found Northern Realms & Nilfgaard were the most powerful and easy for me, but the more interesting one was Scoia´tael.
TL; DR: I voted Monster, but I do think that's going to change...
My first favorite was Northern Realms, but playing with it became too boring. Then I learned to use Monsters and I liked it! It was my second favorite in this game. When BAW got released, now it's definitely Skellige, what a great and exciting deck!
Tried also Elves and Nilfgaardian, but those weren't my thing. I can win with those so it's not about loosing, but still I just don't like them.