I would say that empire size, alignment and research ability are all key factors, trade has some influence too as does the human player. However different races seem to do better then others in the same situations.
Generally speaking and purely based on my own experiences, on various alignment combinations.
Torians are a well rounded AI, they play a part in every game. They are hardy players and manage well in techs and alliances. Overall I have to say they dominate most games, given a bad start they manage to hold their own better. I find them more unpredicable as evil, usually even more dangerous.
Altarians are strong, they like techs and fighting the opposite aligned. I usually play with the Drengins and Altarians opposite alignments, and they seem be 'counters' for each other.
An evil Altarian empire seems to do better as an individual then a good Altarian empire.
The Drengins seem to exhaust themselves in war, unless they get early conquests or start with more planets they will lose. They get big fleets, but are generally struggling tech wise and at war with multiple enemies.
Arceans are unpredicable they can be bad news or good news. Their diplomatic stance shifts a lot, but usually manage an alliance no matter which alignment they play. Otherwise while they can hold their own they take a back seat to the Torians, Altarians and Drengin.
At the bottom are the Yor, they seem to have trouble in fact they seem to play like a retarded drengin. I've never had them become a threat unless they ally with someone. They don't seem to play too badly but then again they never seem to get anywhere. In fact most wars I have with them, they never show up.
Thats generally what I've found.
Overall Torians best, Altarians second, Drengins just behind for third, Arceans forth and Yor alseep.
~SDC~