I've always used the Master Sword. Frankly there aren't enough one-on-one sword fights in these games. You always have either some weird monster that dies in a few shots, some idiot goblin-like enemies swinging random sticks, the occasional archer that runs away when you approach, or bosses with a single obvious weakness. I cherish every proper (or pseudo-proper) sword fight I can get, be it the Iron Knuckle, Darknut, Ganondorf (when he has his swords and is not randomly shooting magic), or Dark Link. To use the Megaton Hammer or Din's Fire would feel too much like another monotonous "exploit bizarre weakness" kind of fight. Link has a sword. Let him fight traditionally every once in a while. Plus, I find it infinitely more fun and rewarding to fight without scale-tipping advantages.