My only problem with velvet is that silk velvet can be extremely overpowering to the point of tacky if it's shiny enough. I think it all depends on how much drape is involved, how deep the pile is, the color, the shape....it can all work to make the dress look great or cheap.
I think the best look is the Donna Karan, it gives the texture and sheen of velvet but it's not too much. The Oscar is pretty as well, since it's very dark and you only get those few glimpses of the true color where the light hits it....but it could just be that the photo captured it really well and it's more intense in person.
The Ralph Lauren, well really all of the velvet dresses in the RL collection bothered me to no end because I truly couldn't tell if it was just a really bad looking silk velvet or if it was velour.