I've been keeping track of some statistics for a few months and assigning power rankings to alliances. Things considered include nations above 100k, nations above 60k, tech per nation, tech per nation above 60k, total nations and military wonders. Here's my top 5 with their score.
MHA 859
Umbrella 843
FARK 601
MK 536
Pacific 531
Note - This is based on purely statistics. Basically, if I could clone myself a few thousand times and control all the nations in each alliance, this is how powerful each alliance is individually. I know these rankings are off realistically but it's still interesting nonetheless.