Again, it will depend on what you intend to use it for. There are 5-minute demos (per day) of each algo at the store found in the H9 Control app. You can get a pretty fair idea of what an algorithm can do with a few demo sessions.
If you can live without the delay feedback, then QuadraVox might handle all of your diatonic harmony needs. It's possible to 'double' your harmonies – say, a 3rd and a 5th on each side – and shift each voice over several milliseconds. Same Diatonic 2-part harmony; thicker textures. Or one-note lead lines that produce 5-note chords locked to a scale. And it can't be beat for cascading arpeggios that can stretch over several octaves.
Diatonic with delay & feedback has its own advantages. 'Only' 2 voices, but the regenerated delays can give the results a finger-picked flavor. I like to swap harmonies or delay times with the expression pedal; then the repeating delays become even more complex. Plus, Diatonic has a unique way of configuring the the feedback length that isn't found in any other algorithm.
Full disclosure: I run a PitchFactor through an H9, so I'll often have one intelligent harmony algorithm running into another. If you program each algo to 'stay out of the way' of the next set of harmonies, it can result in entire phrases based on simple input, and under precise, fingertip control.
