It really depends on whether you have good lighting, use Co2, and what look you want.
EDIT NOTE**** These tanks are not my own.
hemianthus callitrichoides- requires good light and prefers Co2-(picture from-
http://www.linago.hotmail.ru/
Riccia Fluitans- high light, Co2 (picture from-http://www.blueaquarium.org/wp-content/uploads/2009/jan/l_envolee_by_voultou/l_envolee_by_voultou_8.jpg
Dwarf Hairgrass- High light, Co2- (picture from ( I believe this is one of Tom Barrs tanks)-http://i171.photobucket.com/albums/u312/plantbrain/resized70galADAwith1.5wgal.jpg
Mosses would be best if your light isnt strong! ( It would also be the easiest to keep alive) It doesnt really have the same look IMO though.
It takes alot of light to grow ground covers other than moss.