I'd argue it'd be the Taken King Exotic Swords which would be the most meaningful weapons to The Guardian on a personal level.

Whether it be Dark-Drinker, Raze-Lighter, or Bolt-Caster these are weapons who's forging was only made possible by The Guardian's initial victory over Oryx. Sure six of us would have to put the old man down one final time in King's Fall, but that initial defeat was a huge personal victory. Up until that point the defeat of Crota had been our greatest accomplishment, but Oryx made his son look like small potatoes by comparison. Nonetheless the Guardian managed to defeat the corporeal form of the Hive Triumvirate's leader, who is still to this day one of the strongest beings we've managed to best going on six years.

Then we got the heart of his millennia old sword Willbreaker, and forged it into a sword of our own. We took the object that perhaps best represented the Hive interpretation of the Sword Logic, and made it into a weapon for our own use. It's a tragedy that we've seen these swords left behind.