AMD side: Sapphire, Powercolor, and XFX in that order usually. Asrock's been pretty decent as well.

Nvidia? Not really sure, Zotac is a sister company to Sapphire and is usually pretty decent, I've heard. MSI might be okay, but I'd read reviews for the specific card in question. Same with Asus, as I've heard they've been going downhill as of late. Gigabyte's been known for poor quality cards for a while. Maybe not all of them, but more of them than other brands.

That's just the word on the grapevine though, individual reviewers would have more insight. Like Gamer's Nexus and Hardware unboxed.