The Primal 2h sword is the strongest melee weapon in the game tied with the Primal maul and Primal spear. However, it can only be used in Daemonheim. It requires 99 Attack to wield. It can be obtained by defeating Kal'Ger the Warmonger, who requires a level of 113 Dungeoneering to encounter. It is a highly useful bind for those who have access to it, as it can cause large amounts of damage, and can quickly take out slash-weak foes such as forgotten rangers and zombie rangers. Ultimately, whether the player chooses this weapon or its crushing or stabbing cousins, the Primal maul or the Primal spear, depends on which monster weakness they think they will encounter more.
Technically, the Primal 2h sword has the highest level requirements to wield since Kal'Ger the Warmonger can only be encountered at level 113+ Dungeoneering, and you can only obtain the 2h from him, essentially giving it the requirement of 99 Attack and 113 Dungeoneering.