Toothless because he is the fastest dragon, the smartest and he seems to be the leader cause in the series he seems to tell them what to do. And plus he is the cutest!
The obvious answer is the Night Fury but that said, the Nadder is quite a good looking Dragon too. Then there's the Boneknapper which I thought looked pretty cool but based on personality, Toothless wins hands (or claws) down. (Also, isn't Snotlout's dragon called 'Hookfang'?)
