This is an example of an emphatic sentence. They all use the third person of 'bod', ie 'ydy' or 'yw' whatever the pronoun. When we transpose it to English we have to change the form of the verb to be.
Fi ydy'r gorau = I am the best
Ti ydy'r gorau = You are the best
Fe ydy'r gorau = He is the best
Nhw ydy'r gorau = They are the best etc