What in your opinion is the strongest or weakest faction? I've just started playing MO and haven't tried any online games, so this is based on single player Skirmish games only.
Personally I think China is the strongest due to their Nuwa Cannons (which are incredibly powerful against anything on the ground), coupled with their repair drones and EMP weaponry and the Centurion. I find the AI very challenging even on easy, and so far China's the only faction I've beaten the AI consistently with.
As far as the weakest, I'm not sure. I haven't played around with the Yuri factions much yet so it's hard for me to make a judgment call on them. Against the AI I find any of the Soviet factions very hard due to their Kirov airship surprise attacks (which will completely devastate your base if you aren't prepared for it with any anti-air.)