What was/is the most powerful champion or item ever ?

He now exists to end these discussions.

Sion during the beta test had 200% ap scaling on his shield and 100% scaling on his on click 2.2 second stun. Both of these abilities had extremely high base damage on top of their insane scaling along with a very short cooldown. Late game with full build he could instantly do nearly 3k damage to a helpless stunned enemy and take no damage bacause a ridiculous shield that blocked about 2k damage before his passive which reduced the damage he took by 20%.

For items probably Leviathan. It was a stacking item like Mejai's for HP. The cost was 1275 gold and at max stacks it offered 820 HP and 15% damage reduction from all sources.