Hardest fish to keep?

Many say that Discus are hard to keep due to their specific water parameters requirements, but I think there are a ton that can also fall into that due to their diet (such as freshwater puffers, and not the pea puffers) or due to their size as an adult (Bala Sharks for example get massive).

In my experience, Corydoras Catfish (or Cory Catfish) is the hardest to maintain. I bought them because they are bottom-dwelling scavengers and efficient tank cleaners, but maintaining them is really difficult.

Mine was shrimp. I tried them once and they didn't last long. So I am thinking to get from another source and try it out again. Wish me luck!