As an in-home personal trainer, what is your favorite piece of equipment to use with clients?

678 Days later, this question has forced me to change my answer to:
The Heavy Bag! “Wear it Out! I’ll buy another one.”

I also use my Free Motion dual cable cross (w/ separate arms and weight stacks) for evening out muscular imbalances while allowing the client to adapt the weight to their own natural motion. Getting ready to make my rock wall a permanent fixture also.