the mainly people that are op are:
Jax: in various reason bc u add some attack speed and add an excaliber ur just going to die automaticly. even with his stun.
Pantheon: pantheons stun can Lead to total destruction with a spear throw and his jump.
Signed: He Is crazy bc he slows u and throws u back in the pit of mud to get slower and his teamtes will finish him off.
Master Yi: oh man u know how crazy he is with attk speed and wuju/highlander... hes just 2 op and he costs so little.
Last but not least the Ezreal: he can use a barrage that takes out 75% of ur hp when its op just add some ability power and later on if its all the ability powers are the best and u have only but the ability power. it may take out 95-100% hp and will result to a k0
Those are the list of op champions thank you