I have found dwarf/ mini water lettuce (Pistia Stratiotes) the easiest. It thrives in my tanks which are pH 7.6, GH 160ppm, temperature 75f/ 25c. I use a half dose of Seachem comprehensive each week. Maintenance consists of removing the odd yellow/ melting leaf and throwing out excess plants at the weekly water change.
I have tried hornwart, water wisteria, frogbit, riccia flutians, salvinia and eloeda but they did not last. The dwarf water lettuce has thrived for a year or more.