Medium format gear is very heavy, and very, very expensive, but you get maximum image quality and image size. Unless you are wealthy, a working pro, or full time artist, you don't need it. Normal lens is 75 to 80mm.

Full frame format gear is heavy and very expensive, but you get extremely good image quality and image size. Normal lens is 50mm.

APS-C format gear is marginally lighter than full frame gear, if you use APS-C lenses. It yields very good image quality and size. Relative to full frame gear, there's generally a one f/stop low light performance penalty, a one stop extension of depth of field for a given field of view, and a 1.6 crop factor (Canon) or 1.5 crop factor (everyone else). So a normal lens is 35mm.

Micro Four Thirds (Micro 4/3, M43, mu43, etc.) gear is about one fourth the size and weight of an "equivalent field of view" full frame system. It's about 1/3 the weight of an "equivalent field of view" APS-C dSLR system. It yields very good image quality and size. Relative to full frame gear, there's generally a two f/stop low light performance penalty, a two stop extension of depth of field for a given field of view, and a 2.0 crop factor for still images. So a normal lens is 25mm.

It is very hard to purchase a BAD camera these days. All the major brands are quite capable, and the gear is reliable. Nearly all cameras have the same core features, and then a superset of unique features that give each model its particular appeal.

The advantages of dSLR seem to be:

Ubiquitousness — They are the most popular advanced cameras.
Lens selection — An abundance of native and third-party lenses is available.
Best autofocus speed — Some more expensive models are most appropriate for sports and fast-moving wildlife.
No viewfinder latency — You see action in real time.
Best range of models available — see ubiquitousness.

The disadvantages of a dSLR include:

Viewfinder black-out at time of exposure — The reflex viewing system mirror has to move out of the way of light reaching the sensor.
Size, bulk, and weight — Lenses can be heavy, bodies are thick and heavier than mirrorless.
Noise — Shutters and mirrors can be quite loud.
Live view mode requires use of LCD/LED screen on back of camera. This can disturb patrons in a theater.

The advantages of mirrorless cameras include:

Mirrorless cameras are available in full frame, APS-C, and Micro Four Thirds formats.
There is no mirror chamber — you at least have smaller, lighter, more compact camera bodies.
No viewfinder blackout — There is no mirror to have to move out of the way at moment of exposure.
Most dSLR lenses and cinematography lenses can be adapted to mirrorless cameras (with some automation limitations).
Fewer moving parts can mean enhanced reliability.
Lenses can be engineered with better correction of chromatic aberrations, coma, astigmatism, etc.
Micro Four Thirds lenses are MUCH smaller, lighter, and compact than full frame and APS-C lenses.
Many mirrorless cameras have electronic shutter modes that are completely silent. This is great for working in theaters, concert halls, around wildlife...
Electronic viewfinders allow use of the electronic shutter in a dark theater without disturbing patrons.
Electronic viewfinders provide a view of your image the way the camera will process it for a JPEG. You can adjust menu settings and see the effects.
Set the camera to save B&W JPEGs, and also save raw color images for later choice. The viewfinder displays the image in B&W, for easier pre-visualization.
There is never a need to adjust fine focus of each lens to each camera body, as there can be with dSLRs.

The disadvantages of mirrorless cameras include:

Responsiveness — There is still a slight latency (delay) of the electronic viewfinder image. However, each generation of mirrorless diminishes it.
Some folks don't like the electronic viewfinder. But if you haven't tried one in three years, you're in for a treat!
Availability of parts and service — It's still a bit spotty, although Panasonic and others are starting to build professional services organizations and are beefing up their repair teams.

Fujifilm, Olympus, Panasonic, and Sony make the very best mirrorless cameras. Canon and Nikon make the most popular dSLRs. Pentax is still in the mix.

All the manufacturers have at least a decent selection of lenses. Where selection is relatively limited, the available choices tend to be professional quality glass.

Micro Four Thirds is a camera/lens mount system shared by Olympus and Panasonic. There are many lenses available for M43 — http://hazeghi.org/mft-lenses.html

Most mirrorless camera manufacturers have entry-level, "social and travel" cameras, enthusiast-level, and professional-level cameras. Most also have two predominant body styles — rangefinder-like and dSLR-style. Most do not have optical viewfinders. A few only have a rear LCD screen, and no EVF.

You really need to TRY any camera you think you might want to buy, so borrow, rent, or otherwise get your hands on one for a few days. Pay attention to:

Ergonomics — The size, weight, heft, balance, feel, button and dial placement, etc. in YOUR hands, not mine, Joe's, or Sally's.
Menu organization — Some brands have MUCH easier menu structures to navigate. Again, this is a personal preference thing.
Programmability — Some brands have dials and buttons that you can reassign to meet your needs and preferences.

There is no one "best" camera. There is only the one that meets your juncture of budget, weight, bulk, speed, feel, thinking style, photography style, subject choice...

You may want to buy USED or REFURBISHED gear to start. Adorama, B&H, Cameta Camera, and KEH are all good sources of used cameras, lenses, and other photo gear. KEH sells only used gear. Most manufacturers sell refurbished gear through their web sites as well as dealers.

If you are just learning the principles of photography, consider buying a good lens and a cheap, used body. Once you've recorded your "10,000 images," you should know what sort of camera system you really want.

Hi all, I am wanting to buy my first serious camera (had a Brownie Super 27 when I was 7 and took pictures of everything on the planet. Also had a point and shoot enjoyed it too. Now its my cell phone.). I am retired and want to become passionate about photography again. Not sure in which direction to go. I have the intelligence and enthusiasm to learn the working of any camera (also know I need to take a course and do some reading). My plan is to buy the body separately and maybe 2 to 3 lenses separately. I would love to go with a full frame camera, respectable MP, decent viewing screen and one that is enjoyable to learn to use. I will be traveling a lot in the future so landscapes, nature, candid shots of street scenes are my interests. I also love shooting macro or blown up shots of tiny things. I'm 66 and have some arthritis in my hands. First question, would a light weight DSLR be better or can I get decent fast lenses with mirrorless? If DSLR is the way to go, Canon or Nikon? I know enough to know once you commit to one or the other, your married. I dont want to buy cheap now and upgrade over and over again. I'm not 20 so I dont have luxury of many years for that. I want to buy a great camera and gradually add some great lenses, filters, etc. Help, any recommendations?..🤠