Well i would go along with the whole brain thing but since someone already took that answer *cough* dad *cough* i will say my strength to be able to pull a charged line and to use the tools on the truck. I can use my strenght to my advantage.
cheif i know the brain is the best tool but i think you ment which hand held tool and captain bailey obviously wasn't using his brain when he wrote brain cause he put "brian" but my favorite hand tool would have to be the married set its easy to carry and you can carry them while your partner has his hands free, you can use it for forcible entry well with ventilation and well its pretty much just an all around tool... i used this tool alot during training at the fire academy as well in in house training and the interior we had across from the fire house a year or so back on the dirt road in the pouring rain
Maybe Capt. Bailey meant Brian, as in using Brian as tool or that Brian is a tool. My favorite tool is PPE, without it, you will not last long to use the other tools.