German Superlative

German superlative (der Superlativ) forms are similar to English ones. Both languages have ‘(e)st’ endings for superlatives (e.g. fastest / schnellst-), but only English uses “most” for the superlative of longer adjectices. German never does this, regardless of how long the adjective is. In English, we typically use the definite article “the” with the superlative, to express who or what is the most of something.

In German, there are two possibilities: We can, in fact, use the definite article (but remember: genders and cases!), or we can use “am”. 
When using a predicative adjective (someone or something is the tallest, fastest, etc) we can either use the definite article plus the ending “-ste”, or we can use  “am” and the ending “-sten”.
When using the adjective “adverbially”, as in, to describe the quality of a verb, we can only to use “am” and the ending “-(st)en”

Example:
Mein Kuchen schmeckt am besten.  (adverbial adjective describing “schmecken”)
My case tastes the best. 

Mein Kuchen ist der beste. (predicative adjective) 
Mein Kuchen ist am besten. (predicative adjective)
My cake is the best (one).

• monosyllabic adjectives with stem vowel ‘a,’ ‘o,’ or ‘u’ add an umlaut (e.g. alt, hoch, kurz) 
• monosyllabic adjectives ending in ‘d,’ ‘t,’ ‘s,’ ‘ss,’ ‘ß,’ ‘z,’ or ‘sch’ add ‘est’ instead of just ‘st’ in superlative form (e.g. heiß -> heißesten)
• polysyllabic adjectives whose last syllable is unstressed and end in ‘d,’ ‘t,’ or ‘sch’ only add ‘st’ to the superlative form (e.g. praktisch -> praktischsten

Here is a list of common adjectives in their comparative and superlative forms