Superlatives (the Article + Comparative)
The (relative) superlative - “the biggest”, “the best” - is simply the comparative preceded by the definite article: ο μεγαλύτερος (“the biggest”), η καλύτερη (“the best”, feminine), το καλύτερο. Because it is built straight on the comparative you already know, the only new move is to put the article in front and agree it in gender, number, and case with the noun.
The rule
The everyday relative superlative (“the -est / the most …”) is simply the definite article + the comparative, agreeing with the noun: το καλύτερο σπίτι (“the best house”), ο μεγαλύτερος δάσκαλος (“the biggest/oldest teacher”). There is also an absolute superlative in -ότατος (“very / extremely …”), but the article-plus-comparative pattern is the one to produce by default - once you know the comparative, the superlative is free.