If you are a singer trying to compete with Billie Eilish or Justin Bieber, the free VSTs will disappoint you. They produce digital artifacts (clicks, warbles, metallic ringing) because they lack advanced retune algorithms and formant preservation.
For over a decade, was the go-to free alternative. While it is still functional in Audition 15, it is showing its age. autotune vst for adobe audition 15 free