The word "powerful" is a bit vauge. It could refer to max hit, commonly high hits, or special effects such as the miasmic barrage.
The Godsword: Melee and its two handed. It has the highest strength bonus of any non-degrading weapon.
This can be debated, however. Dharok's set effect can hit very high at when you are at low hitpoints. A Black Mask or slayer helm, combined with the Keris on a Kalphite or Scabaras task, or Balmung on a Dagganoth task, can both yield very high hits (100+).
Keris currently holds the max hit ever recorded, at 136.
Certain weapons in the "a soul's bane" quest can hit 200.
Even though it doesn't have a very high hit the ice barrage could be debated as the most "powerful" because it can freeze the opponent(s) making them useless.