It depends on your budget and interests. In terms of sensor size, Micro Four Thirds (from Olympus and Panasonic) is generally the most affordable, but it comes with a smaller sensor. APS-C offers a middle ground, while Full Frame is the most popular and typically delivers the best image quality.

Personally, I use Sony APS-C the most because of its smaller size, lighter weight, and more affordable lenses. Among APS-C systems, Sony and Fuji offer the widest lens selection. Fuji gear tends to be overpriced now, but it does have a stylish look.

Micro Four Thirds lenses are usually cheaper and more lightweight.

If you're shooting fast-moving subjects like birds or Formula 1 racing, Canon and Nikon are the most popular choices. They offer a wide range high performance lenses designed for demanding situations.