Is mexico city the biggest one?
You are the frendliest person i've ever met.
He is the fantasticest player in the team.
They are the proudest parents in the world.
Is she the best singer in the band?
King Kong is the terrifyest film i ever seen.
На счет terrifyest у меня два мнения или как написано или так - terrifiest.
Но склоняюсь к 1 варианту.