Sapphire is the company I purchase from. The do not have the best warranty period, only 2 years. However, on average they produce AMD video cards with that have the fewest problems. In other words, a good product.
I am familiar with and have purchased video cards and motherboards from other manufacturers, however, I have found Sapphire to be the most solid and better product overall.
Also, unlike other companies, Sapphire dedicates video card resources to only AMD products.
To give you a best to worst manufacturer rating would take a little thought, and I do not wish to ponder this enough to do so, I have something else pressing at the moment.