I'd have to say the Golden Sword. Link to the Past was a real ***** of a game and you had your work cut out for you beating the latter chunk of the game without upgrading the sword.
Also, LttP's 3rd sword is simply a refined Master Sword. The Golden Sword actually REPLACES the Master Sword, which is said to be the only thing able to defeat Ganon, so this sword means business. Also, correct me if I'm wrong, but I believe that this is the only game where the highest-level sword is strong enough to take out high-ranking foot soldiers (Zelda's Darknuts still took 2 hits with the 3rd sword, Zelda 2's Iron Knuckles still kicked your ass at level 8, and Ocarina's Iron Knuckles still took several whacks with the Biggoron Sword.)
Well, you don't HAVE TO have the Tempered/Golden Swords. But if you use the blue Master Sword on Ganon, you can only damage him with Spin Attacks. I've done it before and you really need patience, especially doing a minimalist run. In the JPN version, the Tempered Sword can hurt Ganon with regular slashes, but the US version makes you use the Spin Attack to damage him. Also, small bit of trivia, but in A Link to the Past, you DON'T need the Silver Arrows to defeat Ganon. You can simply hit him with a Spin Attack just before he teleports away. Don't believe me? Look it up.
Also in Zelda II, you can OHKO Orange Ironknuckles with your sword at level 7 and Reds go down in one hit with the level 8 sword. As for the Blue Ironknuckles, a maxed sword only takes 3 hits to kill (plus they're easy if you back off after each hit and block their swords appropriately) with a level 7 sword taking 4 hits. Just for the record, Oranges take 8 hits, Reds take 12 hits and Blues take 28 hits from a level 1 Sword. Also, more nerdiness, but the level 8 sword does 12x the damage of the level 1 sword. So in theory, that's the strongest sword since it's 3x stronger than the Magical, Great Fairy and Tempered Swords. Not to mention the Zelda II level 8 sword is 1.5x as powerful as the Master Sword in a vanilla Zelda Classic quest if you want an idea how powerful it is.
Back on topic though, I would go with Fierce Deity Link's Helix Sword. Let's just say Majora has no chance at all if you use it and it turns Gyorg from the boss that makes me rip my hair out and fire a barrage of cluster F bombs to a boss I could probably beat using only one hand.
EDIT: Sorry I didn't realize this thread was grave dug. Please forgive me.
Edited by RedmageAdam, 29 November 2013 - 02:11 AM.