With lights on:
brightness, color, contrast (both good color and contrast are badly diluted, so the order isn't real important)
With lights off:
color, contrast, brightness (close call between color and contrast)
However, if any parameter is really weak, the order changes. If something is "good enough" it seems less important, but if it lousy, it's a big deal.
So, let's make it more complicated:
lights on:
brightness > 2000 L
contrast > 300:1 (ie, noticeably better than Sony 10HT)
lights off:
contrast > 500:1
brightness > 1000 L.
And this is for an average screen size (54x96)
Throw screen size in, and it gets totally crazy.
As Cain said, "It explains why different people choose different PJ's"