Buy 2, front and rear, it can better show what occurred before an accident / agression happened.
Buy one that is easy to operate, easy to use. I have one form a brand called Caliber, and it sucks:
So I'm looking for a new one.
Take the largest SD card that fits the dashcam.
Make sure the time & date are absolute correct, you need that as evidence (in my country anyway)
And buy one where it's easy to transfer the video from the dashcam to your laptop / PC for uploading. I prefer a cable, but most are wireless now I think.
Many dashcams now transfer to a smartphone, but for me that's not ideal.
But perhaps some people find it useful.
And pay attention to where the electricity comes from, traditional cigarette lighter point, or charge through USB or similar
If you but very expensive ones, thieves might want to steal them
