This is a hard one, because in my opinion, all Dragon type suck. They are an overly used (by trainers) and under developed breed. There is much they could do, but they won't, due to possibly becoming too uber.
Because they got too uber in GSC, they for some reason are only making the dragon type teamed with flying and ground. Save legendaries, so that ice at least can whoop them w/o trying much. Take in how many pokemon can USE lessay Ice beam. And ... I pass on dragon type. I'll use them to breed their skills around, but not use them. Hence my dragon breathing Sceptile. lol
BUT to actually answer the post, I would pick Gyarados, if they followed thru on him being a dragon, but course they haven't. I'll go with Kingdra, as I liked Horsea in RBY, but he sucked stat wise once it was done evolving and such. Kingdra brought it back up to being worth training.