Why not Amumu + Morgana? If they're both present, Morgana's ult can do signifigantly more damage (100-150 base damage + 1.4 AP ratio total, compared with Ziggs' 0.9 AP ratio minus 20% damage for enemies not in the very center of the blast) AND stun for an additional 1.5 seconds.
Or Pantheon? Up to 1000 base damage on his ult, with 1.0 AP ratio, a slow, and the considerable strength of his spear to follow up on any survivors. Of course, he doesn't usually go AP, but he's still hitting as hard as a Ziggs with ~600 AP.
If you can keep a team supressed for long enough, Nunu can get Pentakills every teamfight, unless the enemy team splits up so they can die one by one.