ashe's ulti and malphs for initiation,

Best Ult in the game 2 me theres 3 For one Mordes you can take control of the most op on there team for 3 seconds and mordes dmg mixes into them so the auto attacks hurt like heck
Sejuni's 2 can freeze a whole team for bout 2seconds thats nuff time bring basiclly all them to half health amus stun the same gayest ult tho is fiddle its like "whered fiddle go"
"CAWCAWACWCWCWCACWCACWCAQCWCACCACWCWCACWCAW"
"PENTA KILL"

Tibbers!

Right now, for 5v5 Summor's rift I'd have to say SHEN; the spilt push potitental late game to prevent stalemates and turning battle in your favor is super strong

Here's the deal: With ults, yeah a lot of them are good on paper and in concept, but most ults on on upwards of 100 second cool downs. Shen's ult has one of the highest cool downs in the game, it's about three minutes between ults, and does is 850 damage worth of shield at level 16. That is great, but if you are talking about game changing ults, take a look at Mordekaiser's. Sure, it's a % of their health, yeah it's not quite the damage or CC of others, but the potential to turn a team fight into a 6v4 is insane.
I am going to go with Galio's being the best because a 2.5 second CC is better in most of the cases, but don't forget that IF Mordekaiser can focus your AD carry, you will be fighting your amazing AD carry. The problem with it is it's a huge IF.

Galio's in the best.

I'd have to go for Malphite's Ultimate. Not only does it hurt, the subsequent damage reduction, etc is a killer.

Like the a popular saying goes, a team fight is divided into two parts - before Malphite's ultimate and after.