It is impossible to determine which is the absolute hardest houseplant to grow, as this depends on too many factors. For instance, somebody that is living in a tropical environment might not have a lot of problems to grow Calatheas (a tropical plant), while other people that live in colder climates might have a hard time cultivating Calatheas. To sum it up, the challenges that one is facing when cultivating houseplants are very different depending on the climate, the location and various other factors.