For absolute ease: Chaetomorpha. For food: Ulva, Gracilaria, Enteromorpha. For the fastest growers: Caulerpa, in particular Caulerpa prolifera. Caulerpa comes with its own warning advisory, its great in tanks that routinely run high in nutrients (like FO or FOWLR or predator tanks) but has a tendency to self destruct (and cause mayhem) in tanks that run very clean. The others listed will also die off if they do not have enough food, but almost never cause problems when this happens.
>Sarah