So far mine are Wukong and Fiora ult together with Wukong going in first to knockup the entire team then Fiora going in at the same time while hitting every one of the enemies with her ult without them being able to do anything about it.

Jarvan ult + viktor ult = bai bai

Nunu's ult + Galio's ult

Running away? Naw.. just hit me and get blown up twice!

amumu plus kat ults... nasty

amumu and kat ults.... nasty

amumu jungle
kayle/galio mid
jax top
lulu bot
+1 person

need i say more

Here's a couple

Cho'Gath + Darius= Eat1000 true damage
Yorick + Zilean = Our adc is never going to die. Ever.

My favorite has to be Malphite + Galio. Just when they think they're gonna be able to recover from one's ult, the other comes in and wham!

Amumu +nunu+kat+Vlad+Orianna

Amumu jungle, Nunu Orianna bot, Vlad top, Kat mid. Vicious team fight potential.

Jarvan with an enemy malphite, enemy leona, enemy viktor, enemy amumu, enemy annie, enemy anivia, or any other enemy with massive AOE damage.

"I'M HELPING!!!"