This is a tough one since I have several ‘favorites,’ depending on the venue, the exercise goal, and of course the client. That being said, the usual suspects are definitely on my LIST (e.g. TRX, bands, leg press machine).
However if I had to pick a favorite it would be the MEDICINE BALL (one that also bounces). The reason being that if it is heavy enough AND it bounces, I can utilize it for traditional resistance training exercises as well as for power exercises incorporating throws and catches.
This is a very mobile piece of equipment, doesn’t require anything else such as being tethered to a stationary object, and is versatile enough to be used for resistance and power training.