In short, use the default one (builtin_gpu), it's Exophase's one, fastest and most accurate. If you hit bugs (Gran Turismo 2 and Xenogears reported so far), switch to gpu_unai (different set of glitches) or oldgpu (slowest but well tested on PC emus). gpu-gles is experimental OpenGL ES plugin that uses SGX, but it's too unstable to be useful.