I can't see Vader fitting in on either side of the WCW/NWO war. Even though he didn't have the greatest WWF run, I don't think staying in WCW would have been very good for his career in the long haul either. He was never going to be the dominant monster heel after Hogan showed up, and after the NWO started he would have been knocked down another peg or two.
I don't think Austin would have, or could have, ever been Stone Cold in WCW. Stunning Steve Austin was never going to go above upper midcard, especially not with him losing his hair.
Dustin is the only one I think MIGHT have been better off staying in WCW, but then you have to think about the fact that after WCW ended, he would have been going to WWF as plain Dustin Rhodes with the Goldust character never having been created. The odds of him still wrestling on WWE TV in 2015 without ever having been Goldust aren't good.