If you're talking Warranty/RMA wise, HIS, Asus and Gigabyte are probably your best bet. HIS have very vague warranty rules, and use of after market cooler is not specifically prohibited, which is good. However HIS have a silly $40 shipping charge on RMA's.
Sapphire have a £15 charge and Power Colours RMA service is just poor.
If you mean in regards to actual performance, contrary to believe, there are differences in the quality of cards, just very small ones. Over the years, from what I can gather, Sapphire seem to have some of the fastest most OC'able cards. Most likely better builds/components of the same thing. But most of them are roughly the same.
Powercolour tends to be on the cheaper side, though in real world terms it makes little difference. You're pretty much ok with going for any of them, just go with the cheapest, however, I usually stay clear of Powercolor, and go for Sapphire/Asus (Asus usually has better bundles and fan control, but with the 48~~ range their packages are weak). HIS are good too, but usually only when they offer custom coolers (IceQ). Asus generally seem to be on the price side, but if the price is right, you're good to go.
Take that for hat you will. But really, you can't do much wrong.