I honestly find attack to be rather valuable as most of the time my girls just smash the face in of whatever they're facing before it can deal any serious damage to them.

Health and def can be quite good too, but you'll most likely deal less damage and thus have to take another turn where you're hit which somewhat equals it out.

Speed is only important if you need to kill certain things before your fastest team reaches the goal and i tend to not worry too much about it. Sure, they run to the goal faster but who cares about that if you have placed some units correctly, the random tiles mess things up, and i use autorun anyways :P

In the end i guess a lot of kills also depend on whether your skills proc or not.