The very, very easiest is Java fern, IMO. I've even had it growing in unheated and unlit tanks. Tie it onto a piece of rock or bogwood; don't plant it in the gravel or it will rot. Same goes for anubias, don't plant it in the substrate. Anubias doesn't need a lot of light and is quite slow growing.
I find giant vallis the easiest to grow, the twisted one is most difficult with the 'normal' one in between in diffculty. Cryptocorynes are usually quite easy; I've got C. pontederiifolia in my tanks and it grows and spreads very nicely.