Most op champion right now?

Riven does not have the conventional weakness an AD caster has.

Unlike Lee Sin, Riven's damage can stay relevant into the late game just due to her sheer scalings. Unlike Zed, Riven has CC and tank abilities built into her kit.

Her only remaining weakness is that she is melee. Riven can win most melee match ups if the Riven player is good enough. However, if she has to fight someone Ranged, she can be peeled off as she closes the gap.

In a team fight, Riven can't afford to use her abilities to try and reach the other team. If she does, just CC and kill her. Just don't fight her 1 v 1 unless you are playing some late game champions such as Jax, Nasus, or Vayne.