AMD overtook Nvidia at times in the gaming space. I'd say that Nvidia has been king of the hill since the introduction of CUDA, since that's what really cemented their position in the tech sector.
Pre-AMD acquisition ATI also often had better hardware specs than NVIDIA, but their drivers were so often buggy and terrible. By the time they'd been fixed the reviews were long since done on the initial release versions.
AMD seems to run a better software shop, at least.
