again, none.. they are all as good as each other. this isn't 40k were certain armies are more powerful than others.. every ones on an even ground and it all comes down to tactics.
If you're wondering which army we PREFER, it generally comes down to taste and nothing else... I'm playing MI for now because I like models and army. But i think it's be skinnies when they come out... they look really cool..
skinnies would rule the field had they only been given air assets! (Corect me if I'm wrong but they don't get spacecraft do they... ) But they own everybody in front of them especially with nerve beams. I personally play the arachnids because of the cool models but after my friend started with the skinnies I've come to realize their infinite coolness! 8)