Photography is super expensive. There is no getting around that; even $500 is a lot of money, and there is very little at that price point. However, I think I have found the best camera for $500.
Sadly, I paid a lot more than $500 for mine 10 years ago. Nevertheless, the Canon EOS 5D Mark II is an absolute beast of a camera. It was ground-breaking when it was launched, being the first DSLR to be used in TV broadcast filming, a real option for those who wanted medium format film quality (don't, I know it's not the same thing) to jump into digital for, and perhaps the first camera for digital content creators, a DSLR that could do both video and still photography at a high end level. This may seem run of the mill in 2019, but back then, it was groundbreaking. We could shoot the event's video and grab stills on the same ecosystem, changing bodies and lenses around between staff with ease and saving a lot of money. It is also the camera system that I use for all of my YouTube videos and behind the scenes stills in my studio. I will continue to do so until they all die out.
It has great ISO performance up to 1,200 for print and 6,400 for web use, the sensor is very good, it produces great detail, and a large amount of my portfolio was shot using it. I shot gigs with it for years, and even though it has very sub-par AF, I didn't have any real issues. Again, this may seem very under-specced for today's standards, but how often do most of us need above ISO 1,200? I am pretty sure the shot below was at 640 with a slow shutter, so if I wanted the action frozen, 1,200 would have done the job.
The biggest reason that this makes my best camera for under $500 requirements though, is the full frame sensor. Having this for both videos and stills at this price point is unbelievable. I recently used the Canon EOS 90D, and although it has video AF and loads of bells and whistles, in terms of sheer image quality, the Canon EOS 5D MARK II beats it hands down.
What would your $500-or-under camera be?