I have red root floater, Amazon frogbit, Riccia fluitans, salvinia and dwarf water lettuce. I like red root floaters the best because the reddish/brownish colour makes a good contrast to my mostly green plants. I like salvinia the least because some will stick to your arm like duckweed, but less bad because they are bigger plants and most will drop off.
I have hard water and all those plants are doing well. The main thing that seem to cause them problems is strong flow. I used air tube initially to corral them then set them free once there's a good covering.