What is the best sword and how do you get it?

The Biggoron's Sword is obtainable through a trading sidequest. You must be an adult, have freed Epona, thaw King Zora, and cannot warp during timed portions.

Recieve and hatch the Pocket Egg from the Cucco Lady in Kakariko Village. Use the Pocket Cucco to wake up Talon in one of the buildings in the village. Exchange the Pocket Cucco for Cojiro with the Cucco Lady. Bring Cojiro to Grog in the Lost Woods. He will give you the Odd Poultice. Take this to the Old Hag in Kakariko (fenced in building). Take the Odd Potion that the Old Hag gives you back to Grog. You will find Fado in his place. She gives you the Poacher's Saw. Give the Poacher's Saw to Mutoh in the Gerudo Valley (outside the tent). Take your new Broken Goron's Sword to Biggoron at the top of Death Moutain. Take his perscription to King Zora who gives you an Eyeball Frog which must be given to the Lake Scientist in the Lakeside Laboratory. You then need to take the Eye Drops to Biggoron who gives you a Claim Check. Wait three days (or use the Sun's Song a few times) and give the Claim Check back to Biggoron. You will recieve the most powerful sword in the game.

This takes a while, but is worth it. NOTE: You cannot use a shield with the Biggoron's Sword.

The biggorn sword.you go to the top of death mountain and go to the right and there will be a big goron.then talk to him and he will tell you to go get medicen at hylie lake. You get it and bring it back and he will give you like a fossil thing. Then playthe suns song about 6 times or wait and he gives it to you.

A sword called a biggron sword it does twice the damage but the quest is long just seach it on youtube