I don't really think "the bigger the fish, the harder it is to keep". Since from what I've seen, it's just the TANK thats hard to get, and keep. The fish themselves are pretty easy when you got it in the tank, and the filters and everything are doing fine.
So granted, the bigger the fish, the tougher the maintanence, but the fish themselves aren't hard to keep. However, I do agree that keeping a Pima or Sturgeon, or Paddlefish in captivity is the "pinnacle" and the apex of fish keeping. It's the highest thing. But there's also little beats that are just ridiculous. No one has ever kept a Candiru that I know of. Some kind of Tetras are just ludicris and make me want to shake them after they died. I find myself admiring people who can keep Cardinal tetras alive, same goes for Tiger Barbs. Shortly after I wanted them all to die, they did.