What champion should I try in League of Legends?

So, i usually use either Cassiopia or Jax but im not even that good at them. Honestly, i've tried many many champions and still cant find one im good at. Some i've trien and kinda failed at are Ashe, Miss Fortune, Ryze, Master Yi is so-so. I know that people have certain playing skills and I'm hoping that what i said was enough to come up with something.