MSI (at least here in China.) because it offers cheap Tier 1 products. ( $72 for the MSI nVidia GeForce GT440 1GB DDR3 VGA/DVI/HDMI Low Profile PCI-Express Video Card N440GT-MD1GD3/LP over here vs $80 on amazon.)
I can't speak for in the U.S.... but I'd be inclined to say XFX because it seems to offer warranty which covers modding and overclocking.