What is the best piece of fitness equipment to invest in?

Hands down I would tell you that if you could only afford one item it would be a kettlebell. If you are not familiar with a kettlebell, it is one of the most functional pieces of fitness equipment.

Pros
The kettlebell can be used for activities that mimic everyday actions like carrying a bag of groceries, lifting a kid, putting things on a high shelf. With a barbell you are limited to training in one plane of motion, this increases the risk of injury when you go to do things outside of the plane you trained in. The kettlebell does not have this limitation. It allows you to train in multiple planes of motion. Better for long-term training, thus further reducing the risk of injury because it does not allow you to advance quickly before moving up to the next bell. Research shows that inactive adults were able to improve their grip strength with kettlebell training; grip strength is an important indicator of longevity (1).

Cons
Kettlebells can only be loaded so much. For those whose goals involve heavy weights, a barbell would be better. Typically, kettlebell training will recommend you stick with the same weight until you master that weight. 

Fitness is not a race. It is a lifestyle. Everyone has a different goal. If yours is to improve or maintain your health, then a kettlebell would be the perfect addition to your home gym. The kettlebell offers long term training with reduced risk of injury due to the style of training. 

To learn more about Kettlebell training click the link below:
Beginners, START HERE | StrongFirst

Where to purchase a Kettlebell:
Kettlebells: The Perfect Full-body Workout with One Handy Tool (titan.fitness)

References:
1. https://doi.org/10.1186/s12877-022-02958-z