The REAL definition of what a broken champion would be

Fucktons of sustain, Loads of health and resistances, High mobility, Shittons of damage, Absolutely no way to counter-pick, Every single type of crowd control (slow, stun, root, displace, charm, fear, taunt), High siege capabilities, Extreme amounts of zone control, Global damage, Global healing, Insane amounts of armor and magic pen, High survivability, Insane regen in both health and mana/energy, Incredibly high amounts of ad AND ap scaling

For a champion to be truly OP, he/she would have to have EVERY SINGLE ONE of these traits. And no, Riven does not have them all.

EDIT: the champion would also need insane jungle clear and minion wave clear