Janna ult and orianna ult.
Combining a pull in with a push out ult results in enemies splitting in half.

Wukong and Teemo.

Places down a mushroom with the detonation range of Wukong's ult's range. When detonated, it knock-ups in the air and deals damage for 5 seconds before disappearing.

Cho'Gath/Darius: Noxian GuilloFeast will eat ALL of your creeps
Mordekaiser/Master Yi: Children of the Highlander enslaves your entire team
Olaf/Tryndamere: Press R for Undying Ragnarok. How exactly do you stop someone immune to every disable, including DEATH?

Pantheon and Cho'Gath

Grand Skyfeast: Pan'Gath channels for 2 seconds, then jumps to the selected area (range 5500) and deals 400/700/1000 (+1 per AP) true damage to all enemies inside the area. For each enemy killed, Pan'Gath gains a stack of Grand Skyfeast, up to 6 stacks, and gains 90/120/150 maximum health per stack.

Karthus and Daruis - Noxian Requiem: After channeling for 3 sec giant axes come down and hit every member of the enemy team. Does more damage they lower health they have. If any of them die the ult is refreshed.

ahri and lee sin

Dragon's Rush - Lee Ahri dashes forward, every enemy in the way of your dash takes damage is knocked up and outwards from the direction you dash (Imagine Draven's Stand Aside), and the enemy deals damage to all other targets it passes through. You can dash 3 times before this ability goes on cooldown.

Miss Fortune/Darius
Miss Darius uses bullet guillotine and fires several axes dealing true damage and if a champ is killed the CD is refreshed so Miss Darius will be way better pentakiller than darius is =)

Twisted Fate and daruis: Noxian Destineny
Channel for 2sec revile all enemy champs. teleport and deal true damage to any champ. refreshes on kill.

Kassigs
You can teleport anywhere within an obscene range and cause a huge explosion per teleport. Lol wut

Caitius

Caitius aim for the enemy head dealing true damage, refreshes on kill.