The choice is AMD 6970 or Nvidia GTX 580 if you are asking the best card for 3 monitors. If you are asking which brand or OEM of the GTX 560 is best they are very similar because they are all variants of the reference card. I'd look for a good reference model (without the premium for factory overclocking) and overclock it myself using MSI afterburner software. I look for good cooling when I select a card because it is the major factor in OC'ing.
If you want to stick to the same price range and have 3 monitor display for gaming, a 6950 can do the job (although a single card solution will struggle with 3 monitors, FSX might be an exception).