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.