Bill Walsh once said the key to defense was to have talented players. He said you could strategize around a personnel weakness on offense, but that a defensive talent deficiency at any position could always be exploited with good play calling.
The same rule applies in fantasy football, or at least it does early on in the season. After three or four weeks, it will start to become clear which defenses are mastering their schemes, but until then, the safest rule is to put the most physically talented defense in your lineup. I have three such groups in this week's Getting Defensive, along with three others whose talent levels aren't as high as generally thought.