"My favorite pasta with ketchup!"
Translation:Мої улюблені макарони з кетчупом!
Possessive pronouns agree in gender, case, and number with the noun they modify. In this sentence, the noun "макарони" is in the nominative plural, so the corresponding possessive pronoun must be in the nominative plural too, that is "мої."
"Мій" is in the nominative masculine singular (or accusative neuter singular) and would be wrong her.