there aint such a thing as a 'best' gfx engine. if a game uses opengl or dxX depends solely on the choice of the game developers that are used to work with either the one or the other. simple.
both these api's have their highs and lows. d3d is relatively easy in terms of building up the actual engine (the api itself is well documented and ms gives extensive hotline and training support to developers. d3d itself is being developed & updated in close cooperation with both hardware and software developers), while opengl code is based on a cross-platform, cross-company standard, which also means pretty good docu, but *whack* hotline support for game developers. the pitfall here is that when opengl was introduced by sgi in the early 90ies, it was intended to be used in professional graphics hardware and software where rendering speed was *not* the prime mission, so the api lacked and still lacks from it´s relatively weak performance when it comes to consumer 3d. the other side of the medal is real hifi image quality. finally, with opengl its also way easier for game developers to transfer their code to other os'ses capable of rendering opengl such as linux or mac os. with d3d, you´re stuck to windows.
hope you got the point ;))
