The most common one is
Anubias barteri, if I'm not mistaken. I think about 80-90% of all the Anubias sold in my area belongs to that species, and with reason: it gets quite large (10-14" high, depending on the variety), it's hardy, does well with very little light, and can also thrive in tanks with just a small amount of gravel (it doesn't have large root systems, like many other plants, and can be attached to a piece of wood or rock with some fishing line or nylon thread).
Above applies to most other species as well, but they are usually more rare and expensive.
Apart from two or three pieces, all my Anubias plants are A. barteri.
Pictures