This is what I'm observing in my games.
It seems that, assuming the difficulty and personality of the AIs are similar of the same, that the races who comes out the strongest and those who come out the weakest tend to change from game to game.
For example, in one of my TA games, the Drath Legion is my biggest enemy while in another, they die out quickly.
The name in that sentence is changed often. Sometimes, the Torian Confederation came out on top. Othertimes, the Drengin Empire came out on top too. The list goes on.
In every game, does the game set an AI race that will become your biggest rival in terms of power?