This question is unfortunately kind of subjective... Best at what? Battery life? Feature set? Value?
In all honesty... I would first decide what I wanted the smartwatch to do for my particular scenario and then pick the device that best suits those requirements.
Without knowing this information, I would really just be throwing out a general answer that I like, but may not be best suited to your particular use case.
That said, my generic answer without knowing anything other than you want a good smartwatch that works with your Samsung would be the Asus ZenWatch. I use one as my everyday watch and is currently my favorite.