Varus has an extremely good ultimate for team fights. Team stun ftw.

In my opinion, it's Amumu's

Its hard to pin down a specific ultimate as being the best, as so many ults can be showcased by certain team comps. for example, a well timed nunu ult with the right team comp is really hard to beat. However, If I would have to put it as this.

Best Overall: probably Karthus.
Best Chase: Twisted Fate
Best Escape: Hecarim/Malphite (any character with a flash like ult in general)
Best Teamfight:Amumu

And my own category
Best Laning Phase: Shen

Also I'd like to give a shout out to Orianna's ult, as it synergizes so well with a bunch of different champs, such as malphite, miss fortune, etc.

For CC--Amumu, Galio, Nunu, Rumble, Zyra
For Damage--Karthus, Katarina,
For Annoyance factor--Teemo, Mordekaiser, Kayle, Zilean, Shen, Urgot

For me:
Best CC: Amumu, Nami, Galio, Zyra
Best Damage: Karthus, Evelynn, Katarina, Rammus
Best Chase: Nocturne, Shyvana
Best Escape: Evelynn, Shyvana

AOE ult team comp:

Top Lane: Rumble, Malphite
Jungler: Amumu, Malphite
Mid Lane: Orianna, Karthus, Cassiopeia, Anivia, Lux
AD Carry: Ezreal, Graves
Support: Nami, Sona, Leona, Zyra

Xin Zhao's ult is great for escaping from enemies chasing you, knocking back enemies chasing your ally, or, if you challenge an enemy with your passive, it will do more damage to that enemy(but won't knock back the challenged enemy)

I really like Maokai's ult, depending on placement in a team fight it can make or break your team.
Damage redux for your team inside ult, good aoe damage after some fighting inside.