I have used riccia, duckweed, hydrocotyle and hornwort. Of these, riccia and hornwort are the best at providing cover, but they also both tend to shed a lot.
I have a stem plant called milfoil in my sorority tank, which grows like a weed in the right conditions. I took mine out the other day during a sorority re-haul and some of it measured at least a metre in length. As you can see in this picture, it coils up against the sides and top of the tank and ensures lots of hiding places for my fish.
(Old shot of my sorority before it really grew in)
As long as you have the correct lighting and dose at least some basic ferts, all the above plants are really
easy to grow. If you want something really basic, you can use a cork covered in java moss. As the moss grows down into the water, it will provide lots of cover for your females.