Not sure if this counts but the Golden branch from Tutorial Island was one the heaviest item in the game but you can no longer obtainable. and also disappeared after a few mins. These are just some items not corpses.
it weighs 1000 oz.
Otherwise I guess a stone can be picked up which large ones weigh in at 780 oz.
Other heavy items that I have had a hard time picking up Include:
helmet of the deep at 210 oz.
heavy package from the Traveling traders quest which weighs in at 600 oz.
letterbag from the Postmens quest weighs in at 500 oz.
Ahmet's Stolen property bag From the steal from thieves quest which is 400 oz.