Which Ambient Occlusion Is Best?

HDAO is more subtle than SSAO and HBAO. It's probably the most accurate, because there's much less incorrect darkening. Choosing HDAO over SSAO or HBAO results in a small framerate drop on AMD cards, and a significant one on Nvidia cards.

What ambient occlusion should I use?

The most common type of ambient occlusion is SSAO or screen-space ambient occlusion. ... Apart from SSAO, there are also HBAO (horizon-based ambient occlusion) and HDAO (high-definition ambient occlusion). These two technologies belong to Nvidia and AMD, respectively, and thus perform better on their own graphics cards.

Should I have ambient occlusion on or off?

You would want to use ambient occlusion because it shows subtle variations in lighting and helps your eyes detect surface details that would otherwise be washed out or unnoticeable. Ambient occlusion is great for softening the overall lighting in your scene if it's too bright.