"Which bag is the heaviest?"
Translation:Welke tas is het zwaarst?
"de zwaarste" focusses more on it being an object, the heaviest of the bags "het zwaarst" stresses more it being a superlative.
Hope this explains things a bit.
Both are possible, but I have seen/heard "Welke tas is het zwaarst?" in use more often. There is no (substantial) difference in meaning.