Titus 1:12 quotes one of their own prophets saying Cretans are always what?

Answer: Liars

A prophet of their own said, "Cretans are always liars, evil beasts, lazy gluttons," and Paul says this testimony is true.

“As one of their own prophets has said, “Cretans are always liars, evil beasts, lazy gluttons.” This testimony is true. Therefore rebuke them sternly, so that they will be sound in the faith”— Titus 1:12-13 (BSB)