Only going to comment on one brand because I have tested it, when my Kingpin was wrecked I hit the Pavement Hard .. Shattered Ribs , Broken Leg, and just plain banged up from the impact .. Although my Shoei Full Face Looked like someone had beat it with a Sledge Hammer there was no Damage to my Face or Neck .. Plainly the Helmet withstood 50 MPH Impact without breaking, even the Face Shield though cracked all up never broke ..
The absolute worst helmets are the Beanie style that so many think look cool but actually look very stupid.
Several things to consider,
Dress for the crash
Protect your hearing, ear plugs etc.
If you have a $10.00 head, then use a $10.00 helmet.
There are several good full face flip up helmets out there, I and my wife use Schuberths.
Make sure your helmet is properly fitted
Shoei, Arai, Schuberth are among the very good helmets.
DOT approved is good enough. Even my cheap Fulmer modular kept me alive after a 70mph crash 3 years ago that I'm still having surgeries as a result. Major concussion from the impact and the modular part nearly in 2 pieces but no lasting problems to head. Except maybe it knocked any sense to not ride again out. I was back riding the first week out of rehab. Now my leg is another story.
This may not need to be said, but better safe than sorry.
Do not buy a used helmet unless you absolutely trust the seller with your life.
I wouldn't buy a helmet from my brother or father.
You really have no idea what it has been through on the outside. Often you can't see if the helmet has impacted something.
The interior has molded itself to someone else's head. It probably won't fit yours very well.