Who is the Tallest?
When using the superlative, care must be taken with regard to the gender of the thing or person being singled out as possessing some quality in the highest or least degree with respect to the other members of the group being compared. Let's stick with Roberto for now:
Roberto es el más alto de los tres. Robert is the tallest of the three.
Notice two things about the structure of a superlative statement. First, the use of the article el, la, los, or las is placed before más (or menos) and it will agree with the subject (in this case, Roberto, and thus el is used). Secondly, the preposition de is used, not en (as you'll see in a moment, English can use in or on when forming superlatives).
Here's another example to show how the gender of the subject can change and therefore the article needed. Notice too how the English translation uses in and not of:
Juana y María son las más inteligentes de la clase. Jane and Mary are the smartest in the class.
When speaking of teams, English speakers frequently forget that Spanish consistently used de and not en, so watch out for that. It's an easy mistake to make but it's just as easy to avoid if you memorize an example:
Pepe es el más rápido del equipo. Joe is the fastest on the team.
Finally, the irregular comparatives -- mejor, peor -- are used to form the superlative when best and worst are used in English:
Ella es la peor estudiante de la clase. She is the worst student in the class.
Tú eres el mejor amigo del mundo.