Sigma
If you're just starting to set up your camera kit, you face a baffling array of choices: fixed-lens or interchangeable? DSLR or mirrorless? Nikon, Canon, Olympus, or Sony?
Most people don't need to buy expensive equipment, since smartphone cameras have gotten pretty damn good. It's probably more beneficial to spend your time learning about lighting and composition than worrying about the differences between APS-C and FX.
However, if you're ready to get serious and invest in some solid gear for shooting weddings, portraits, or just your family vacations, the real magic is in the lenses.
Buying a fancy DSLR or mirrorless camera and outfitting it with a cheap zoom lens is a mistake - the differences between cameras are incremental, and most working full frame DSLRs sold on eBay will do perfectly well as you're getting started. However, a solid arsenal of prime (non-zooming) lenses will not only give you the best quality images, they'll also force you to develop better habits and be more creative in your image-making.
After cycling through a number of primes in the last five years, I've landed on two that I'm convinced are the only ones any serious photographer needs: The Sigma 35mm f/1.4 and 85mm f/1.4 DG HSM "Art Lenses."