It's hard to tell which one has the best visual quality however like one of NVIDIA's engineers said on a recent interview (posted on the frontpage news) Gainward cards have got their complete approval.
I'm also using Visiontek's Ti cards and they look good to me, better than previous cards for sure but still not Matrox rocking 2D, my guess it's that Visiontek is also using high quality components because they similar cards to reference boards, so, there you go...