"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
When adjectives (such as snel) and their comparative/superlative forms are used in predicative position (i.e., after a copulative verb, such as zijn) and no other noun is following the adjective, we don't add the -e- to it and we use het as an article.
Compare, for example, the original sentence with the variation I came up with to illustrate this point:
Hij is het snelst. (He's the fastest)
Hij is de snelste jongen. (He is the fastest boy)
Hope this helps.