Most Icelandic adjectives form the superlative with -ast (e.g. fallegur → fallegastur, fallegasta, fallegasta). However, góður (“good”) is irregular:
• Comparative: betri
• Superlative masc. nom. sg.: besti
• Superlative fem. & neut. nom. sg.: besta
Since mynd is feminine singular nominative, we use besta.
This is a linking (copular) sentence structure:
• Myndin = subject (“the movie”)
• er = verb (“is”)
• besta mynd ársins = predicate nominative (“best movie of the year”)
The first Myndin tells us which movie, and the second is part of the descriptive phrase.
Yes. Adding sú or hin (“the one [fem.]”) makes the predicate explicitly definite/emphatic:
• Myndin er sú besta mynd ársins.
Both versions are correct; sú/hin simply reinforces “the” in “the best movie,” though the shorter form without it is more common.