If running 227i community patch: Use OpenGL. It's the main focus of development in that patch, and traces it's roots back to the excellent UTGLR renderer. The enhanced D3D9 renderer is still pretty good, and should be used instead of kentie.net D3D10 with 227i if OpenGL is not working for you, for some reason.
If running 226 and willing to install a third party renderer, use kentie.net D3D10. It supports all features, has no known bugs and displays a "correct" image on all graphics cards.
If you want a "fully vanilla" renderer, use a glide wrapper (nGlide is good) and run with the Glide renderer.
In vanilla Unreal Gold, the Direct3D renderer still has many graphical glitches and missing features--masked textures are often displayed incorrectly, the game may run too fast on modern CPUs, skies lack multitexture effects and uses OneXBlending resulting in a darker, muddier look to the game compared to other renderers. In short--avoid.
Same goes for the stock OpenGL renderer, it's missing features and incredibly buggy (in fact it may simply crash if you select it)
Post edited April 15, 2014 by xenobrain