For a real time harmonizer, all these years later, it's still Eventide Quadravox/Octavox. It's not going to replace tracking actual harmonies, nothing will. But does let you pick the key for it to transpose to. Not just the interval but the key as well. I don't know why other plug-ins don't have that option. Tracks fairly well.

Otherwise, non real-time, Melodyne or manual pitch/stretch.