Technically graviton Lance. It fires mini blackholes. If the game was true to lore then it should 1 shot anything and possibly destroy existence since blackholes tend to grow and expand. As I recall a blackhole the size of a baseball would be enough to destroy the earth.

Telesto

Scorn crossbow for sure. Or the Architects /jk But in all seriousness, depends on how you define most powerful, and weapon I guess, and in what setting. Weapon as in any weapon? I mean, the Almighty literally devoured planets for fuel in order to blow up stars and therefore whole solar systems. The Cabal Light suppressing tech was so feared that word of its rediscovery was suppressed by the Hidden in case the Lightless ever found out about it and potentially use it against Guardians. Oryx's Dreadnaught. Enough said. Also, the Harbingers I guess too. Weapons in lore? Wish-Ender was used by Sjur Eido, a mortal Awoken, to heavily wound and then kill Shaxx. Through armour. And he thought it was a ballista, but was stunned to find it was a mere longbow. She also killed Ahamkara with it. (In game, it is one of two weapons specialised to fight Taken, and the only one not based on the Taken themselves. It is also the only thing capable of harming Riven's corrupted eggs) Oryx's Sword was literally sharpened on the death of untold billions. By extension, because we shattered that blade and incorporated it into our first line of 'new' Sword technology (Raze-Lighter, Bolt-Caster, Dark-Drinker), does this make those swords the most powerful? Xivu Arath's Blade would fit in a similar vein, but the Witness was able to dissolve it to ash when they last spoke. The Weapons of Sorrow, like Thorn, are known to permanently kill Lightbearers. Then I suppose, in game cutscenes: The Scorn Barons were able to destroy Cayde's Ghost with a Thorn like bullet, specifically crafted for the occasion to the point only one of it existed. Ghosts can normally only be destroyed by paracausal means, weapons or paracausally enhanced beings (under most circumstances) Saint-14's head(butt). Also supported in lore. SIVA. Also supported in lore. The Witness itself could be considered a weapon, I suppose. In that vein, could us Guardians ourselves, be considered weapons? Powerful because they defined the meta for a long time in game? Prometheus Lens, Lament, Mind Bender's Ambition, Felwinter's Lie, Lord of Wolves, Divinity, Hard Light, Anarchy... to name a few. Powerful because they broke the game? Telesto. Always Telesto.

Controller sniper probably….