Best: I'm probably gonna go with Aquaman for the best as well. He has a great trait for nullifying a lot of rushdown heavy characters, long range and fast normals, decent mobility, great damage output, fast multihitting specials, projectile immunity special. decent spacing tools, and a godly D2. His biggest downfall is his lack of a reliable wakeup, but this downfall can be avoided with efficient use of his trait.
Worst: Not sure, I don't know enough about Zatanna to weigh in on her being the worst. Maybe Ares? His mixups tend to be predictable (personal experiences, I suppose), short range normals, poor mobility, lack of anti-airs, and poor wakeup options outside of his teleport, which again, tends to be predictable.