I've owned various brands of ATi cards, and never really had a problem:
ATi themselves... back when they were the only ones.
Sapphire, since they're *the* ATi OEM to me, I believe they've been around the longest, and have the strongest ties to ATi.
Club3D, because they're usually the cheapest you can find here, and they deliver solid reference-design cards for both ATi and nVidia.
Asus, also solid reference-design cards, usually with a good overall bundle of software and hardware. But you generally pay a bit extra.