Easiest plants to keep?

[blu the betta]

Water wisteria is easy to keep. if you get them at petsmart the small plant tubes have 2-4 plants in them. the small ones are about $5.00 i would get the small ones. they are much cheaper. the large ones are about $8.50

[Laki]

Nobody else mentioned salvinia. As a floater it is closer to the light and grows like a weed. The roots hang down into the water and overall they're just great. The roots are fuzzy so the bettas like drifting through them and if you have shrimp they like to graze off the infusoria in them.
Protip, if you ask for a small sample for free you will end up with more than you can possibly deal with!! (this is how I got mine)

[emeraldmaster]

As an off type. Algea is actually a easy plant. It works just like all the others, just spreads like fire. But if you get an algea eater, it is easy to keep controlled. And it is free, people just don't like it!