Hard to say-- here's some of my favs:
Java Moss: Grows super fast, clings to anything, needs little light, soaks up tons of excess nutrients, and looks cool.
Java Fern: Way too many reasons to love java ferns.
Anubias nana: Slow grower, needs little lighting, frames tank well.
Hair Grass: Good for putting highlights in the foreground of the tank, good for foreground, good for holding riccia down, just good.
Micranthemum Micranthemoides: Beautiful plant, can be foreground, mid ground or background in a small tank.