Are you invested in one of the current battery platforms now? I've got a Milwaukee M18 5" angle grinder & I've very happy with it along with their right angle die grinder & straight die grinder. These days they are all comparable so I'd go with whichever battery platform you are using now. If you are not on one & don't plan to get other cordless tools, any of the top common brands will be good with minor feature differences. If you do plan to get more cordless tools, look at the brands to see what other tools they offer that might interest you.
I used use Dewalt for a while. My hammer drill & charger got stolen at work which bummed me out. My company offered to pay for a replacement. I then switched to Milwaukee & couldn't be happier with all the tools I own now.