Duckweed is great for absorbing ammonia and it's pretty! I think it's good all around. I got my duckweed from a shipment of anacharis plants. There must have been a piece of duckweed in there because one appeared floating one day and now I have tons!
Also, if you have trouble with it being sucked up by the filter, I recommend using the Fluval Pre-filters that fit on the intake of the filter. They're really wonderful because they hold beneficial bacteria and it keeps your betta's fins from getting sucked in and torn up.