Using Keris, a player with 99 strength could hit up to 117 on assigned Kalphites, using the best strength equipment, the Piety prayer, a black mask , and drinking a super strength potion.
In the Dungeon of Tolna during A Soul's Bane quest, players could hit up to 200 on monsters in the rage room, however that damage is simply a player's normal damage times twenty, and doesn't give any experience.
When touching the Stone of Jas, all the combat stats excluding prayer will be boosted to 255. Max hit unknown in this case currently. Theoretically it is 210+