Do I have to pick just one?!
Okay. It used to be wisteria, as possibly THE most useful beginner plant. It eats ammonia, floats as a betta hammock, can be planted, and sheds leaves which grow roots and become baby plants.. brilliant stuff.
BUT. I was recently introduced to Hemianthus callitrichoides,
or 'Dwarf baby tears':
And I adore it!!! Such a cute little ground cover plant. If I manage not to kill it, it may just be my new favourite.
It tends to float, so planting can be a bit of a bother.. basically, shoving it in the subtrate with a few leaves poking out will get it to stay down and put out roots.
In a low light setting it will grow a bit leggier than under stronger light, but is still very pretty as a foreground plant. It does like fertilisers, but I'm hoping the soil base of the NPT will give it what it needs for now. :)