The Dragon weapons are easily the best in the game. You need level 60 Attack to wield them and must complete certain quests to use them. The following is a list of the quests, and which weapons they allow you to use:
Lost City: Dragon Dagger and Longsword Heroes Quest: Dragon Battle Axe and Mace Regicide: Dragon Halberd Monkey Madness: Dragon Scimitar None: Dragon Spear
Note: The Dragon Spear is not quite as strong as the halberd, but is the rarest weapon in the game and therefore costs 1,000,000 gp. However you do not need to complete any quests to use it. The easiest way to use Dragon weapons is to complete the Lost City quest.