You've gotten lots of good responses here, but one thing I'm going to disagree with is the tank focusing on the strongest. In a proper group, the tank should only need to hit the elite/champion mob with a single high-threat attack to have aggro on him, and should then basically follow the DPS to the silver/strong mobs to get and hold aggro on each of those as the DPS focus them down. That assumes the DPS are following an established kill order (weakest to strongest, left to right for example).
That makes sense if there are multiple mobs that take awhile to kill, yes. I generally don't bother using single-target attacks on anything below strong when I'm tanking, since a single geared DPS can take a standard enemy down in about three hits. Conversely, DPS should watch what the tank is doing and attack the same target when it makes sense with regard to the kill order.