"He is the fastest."
Translation:Hij is het snelst.
From the wikipedia article linked in different question:
... With the article de, there is an implied noun, and it might better be translated as "the most expensive one". The superlative must also be in the inflected form in this case, de duurst would be incorrect. With the article het, there is no implied noun, and both the inflected (het duurste) and uninflected form (het duurst) can be used.
As far as I understand there is no difference between inflected and uninflected form in this case. Further read http://en.m.wikipedia.org/wiki/Dutch_grammar#Comparative_and_superlative