What's the best affordable under $500 dual suspension mountain bike ?

Second hand.  Its your only hope.  Here is what you want for a good susser:

Air forks

Air rear can

Lightweight frame, well made frame.

Hydraulic discs 

Branded components, like SRAM, Shimano, etc.

For the fork and rear can you should look at manufacturers like Rockshocks, Fox, etc.

All that said is going to cost you at least 2000 on a new machine.

https://www.giant-bicycles.com/us/anthem-29-2-2021

Even on ebay you are not going to get a susser for what you want to spend.

Here is the problem that you face with your shopping list and budget.  You might get something that looks like a mountain bike, but you will find cheap gears and parts.  A heavy frame using something like 7005 grade aluminium, as opposed to 6061.  The suspension is a re-purposed bed spring.

In short, your bike will try to do too much for the budget.  The solution is to do it on the cheap.  You will have a heavy bit of junk.

Best option.  Hardtail.

Luck

For under $500, there is no best dual-suspension. At that price point, suspension components will be crappy, and likely the rest of the bike will be too. Try pricing out name-brand suspension components and you'll see what I mean. For under $500, you're better off sticking to unsuspended.

Don’t waste $500 on a dual suspension mountain bike. You’ll get a heavy frame and poverty spec components. It won’t even get you a decent hard tail.

A good source of good value bikes are trail centres where they hire bikes. They will be a top brand and They normally sell them after a season and they will have been well maintained.

You still won’t get a good dual suspension bike but you might get a good spec hardtail .

No doubt, someone who knew what to look for in a mountain bike could find a smokin' good deal on Craigslist. It might take a few weeks for a good one at a very good price to turn up and, obviously, he would have to check it out in person to be sure it was the one. Unfortunately, someone asking too-simple questions on YA is not that knowledgeable person.

I hate to disagree with my esteemed colleague from the Denver area.  But, anytime you buy a used bicycle (especially a used full suspension MTB) you'll end up with someone else's headache.  With all the use & abuse a bike like that goes through, it won't be perfect or even close.  Be prepared to drop a lot more money on repairs & upgrades.   

There are simply no new dual suspension bikes at anywhere near that price. You'll have to go second hand for anything even vaguely decent, and at that price you'd probably get something pretty battered that would probably need a good service - which would cost.

People were riding mountain bikes for decades before they ever heard of the term suspension associated with bikes. Somehow they were able to do it just fine.On rough terrain simply moving your rear end up slightly from the seat will allow you to avoid the shocks that would have been transmitted to your body. All that you really need is a bike with a front suspension, and practically all currently manufactured mountain bikes come with front suspension, since it would be difficult to keep your hands on the bike and control it while avoiding the road bumps. But even keeping your elbows slightly bent can help you avoid a good deal of the shocks. Just make sure the seat you use is well padded so you do not have to rise up from it as often. All you give up is the ability to pedal nonstop even over very rough terrain. Just rise up and stop pedaling for a few seconds when the terrain is extremely bumpy. 

You need to shop on Craigslist in the cities that have a lot of bikers.  Set search option to 450 to 500 and see what you find.  This was 500 in Denver for example not great, a bit old, but xtr is serviceable and its fill suspension.  

I had to look twice.  Then, I nearly fell on the floor laughing. 🤣  $500 MIGHT get you an "entry level" front suspension only MTB.  Here's one from Fuji for $399.99. https://www.fujibikes.com/usa/bikes/mountain/sport...  It uses a cheap (inferior) Prowheel crankset & an equally inferior Shimano freewheel, 14-28T - instead of a better "cassette".  

A truly "GOOD" dual squish bike starts around $2,000, not $500.  https://www.fujibikes.com/usa/bikes/mountain/trail...

Your question is basically an oxymoron; two or more things in the same sentence in direct opposition to one another.  "Best" and "under $500" when it comes to full suspension MTB's simply isn't going to happen.  The only full suspension bike you'll get for that price will be a piece of junk to start with.  

Haven't you seen this YouTube video?  

https://youtu.be/yotOZVELSMc