This question was posed to Dr. Michael Lam, a doctor famous for his crusade against Adrenal Fatigue.
Answer: “The best piece of equipment is perhaps the cheapest – the jump rope. It offers cardiovascular benefits and tones the upper body at the same time. Nothing else comes close. Practically speaking, however, the best piece of fitness equipment to purchase is one that fits into your budget and one that you will use regularly.”
I personally use the Buddy Lee Master Jump Rope.