The one in the Korg PA4X works best for me, and is mostly automatic.
I prefer the Harmony M to the new VH K because of the simplicity of operation, however I don't hear a difference in sound. You can access many more functions on both via software, but I've never done that.
Most of the time all I want is one voice above mine.
I've never been please with the Yamaha harmonizer, although it is usable if you spend enough time setting it up. The Ketron is very good. The more recent Rolands don't have one.