Well, I’m glad you asked! When I was first starting out, I was able to try models of all the big three (Canon, Sony, and Nikon) and just IN MY PERSONAL EXPERIENCE can try and give you some direction from making that jump from hobbyist to professional.
First thing you need no matter the brand: A Full Frame Sensor. To put it simply, a full-frame camera resonates best with the phrase “what you see is what you get”. Meaning, that if I take a 50mm 1.8 lens, it’ll shoot at 50mm, where a crop sensor has a 1.5x field of view on any lens attached to a crop sensor body, making that 50mm lens, a 75mm lens. This usually leads to lens distortion and vignetting as well. So first step when looking at a camera? FULL FRAME.
An early image from when I was starting out with the Canon Rebel T3i (with a nifty 50mm 1.8).
Now to dive into what I think your first camera should be. My biggest two factors for loving a camera are it being user-friendly and the quality of the photo. That's why in terms of a perfect entry-level I look at the Canon 5D Mark II.
When starting out, I had the rotation of a Nikon D3500 and a Canon T3i Rebel. The Canon off the bat was just easier to use. They simplified the amount of buttons on the body and made it super simple to make minor adjustments as you were starting to play more in manual mode. The Nikon’s user interface to me was simply just too confusing to try and play with Manual mode and the color science on Canon is just simply amazing. Making the jump from the T3i to the 5D Mk II was seamless and easy.
Now to speak more about the color science, that's where we can talk about the battle between Sony and Canon. I currently shoot with both a Canon 5D Mk II and Sony A7rii, both full frames and both great cameras in each respect. However, for someone looking for that entry-level professional camera, edge again goes to the 5D.
Pictured Left: Sony A7rii vs Pictured Right: Canon 5D Mk ii
Again, the colors are simply more vibrant and accurate in the canons, but what I feel that is sometimes overlooked is actually the file size in Sonys. For example, in my Sony A7rii, each picture taken in RAW format is 42 megapixels compared to 20mp in the 5D and 12mp in most entry-level DSLRs. Now while you might think bigger megapixels mean bigger and better pictures, I don't find that always to be true. Especially when starting out and dipping your toes more and more into post-production and editing, bigger file sizes can end up slowing your computer and leaving you frustrated. Why not save yourself the headache and get yourself a camera that will EASILY get the job done.
Again, these are all just my opinion. I’ll now wait for the flood of hate mail from Nikon and Sony die-hards to soon hit my inbox.