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.
A good reliable heavy bag! I have never seen anyone work so intensely as when turned loose on a heavy bag for 2-3 straight minutes! Young and old, male and female, all are equal against the everlasting opponent. Really LOVE the Free Motion Dual Cable Cross too though… and kettle bells.. oh, and medicine balls.. and suspention trainers, and oh yeah, .. wait… what was the question again?
I would have to agree with a few of the othere responses, TRX is my favorite. You can do hundreds of exercise with the TRX, can be used anywhere, and can be modified for all fitness levels. In a tough tie, resistance bands and dumbbells are my 2nd favorite, and for similar reasons to why I love TRX.