My take
I am assuming the nations are handled by expert players. Since some nations are better in the hands of a master :
I'm also assuming a large MAP with more the two thirds land mass.
My list is based on MP experience only.
And I admit I'm a bit biased towards Pythium. (Love makes us blind :
Pythium
Caelum
Abysia
Vanheim
Machaka
Ry'leh
Arco
Marignon
C'Tis
Ermor
Jotun
Pangaea
Atlantis
T'ien Ch'i
Man
Ulm
Mictlan