The superlative of the adverb differs from that of the adjective explained  above. It is formed as follows:

langsam  am langsamsten => slowly  slowest
schnell  am schnellsten => fast  fastes

There are several irregular adverbial superlatives, e.g.

gern  am liebsten => like  like most of all (see gern/lieber above)
gut  am besten => well  best
hoch  am höchsten => high  highest
nah  am nächsten => close/near  closest/nearest
viel  am meisten => much  most of all

This is how they are used in practice:

Er ist am schnellsten/langsamsten gelaufen.
He ran (the) fastest/(the) slowest.

Wer ist am höchsten gesprungen?
Who jumped (the) highest?

It can sometimes be difi cult to ascertain whether an English superlative is the superlative of the adjective or the adverb. The test is to ask yourself if ‘the’ before the superlative can be omitted and still sound correct, in which case you are dealing with the superlative of the adverb and thus an am …-sten form is required; if it can’t be omitted, you are dealing with the superlative of the adjective and a der/die/das …-ste form is required, e.g.

Wer hat am besten gesungen?/Wer war am besten?
Who sang (the) best?/Who was (the) best?
Wer war der beste (Sänger)?
Who was the best (singer)?

Although this distinction can be a little tricky to determine in English, German offers you an easy way out. The am …-sten form is very commonly used as the superlative of the adjective, as well as of the adverb, and thus it is seldom necessary to make any distinction if you stick to the am …-sten form, e.g.

Wer war der beste/schnellste/klügste? or
Wer war am besten/schnellsten/klügsten?
Who was the best/fastest/cleverest?

In forms like der beste/schnellste/klügste the adjective is not capitalized, despite the fact that it would seem to be functioning as a noun; it is felt here that the noun is implied.