What champion defies item diversity the most?
Champions that defy item diversity make it impossible for certain items to be introduced without breaking the game or forcing a rework on those champions. For example, if they made the new AS jungle item more of a fair rush as all of the other ADC items ((That item is pretty terrible for pretty much every champion.)) then Kog'Maw would be instant banned because he would be doing too much damage.
What champions do you think cause too much damage to item diversity?
for assassin type items, when the 
came out he was and still is banned a lot simply because of the double ult damage he can cause with it
for mage type items, because she can stun you into oblivion with 3 of her 4 abilities and any high end ap item just makes her that much more annoying to deal with