Raw is the flagship show. It was the first one. It was the one that was embroiled in the Monday Night Wars against Nitro. Without it there would be no Smackdown.
However, for a time Smackdown was the better of the two shows. It wasn't even close.
It was sad how SD was kicking Raw's ass in terms of entertainment value. The storylines were better, the talent made better use of their tv time by delivering better matches, even though they were supposedly 'less talented' than Raw talent. I 'd skim through Raw but would never dream of missing Smackdown. Performers would do great things on SD only to go to Raw struggle and be released. It seems for some reason WWE just hated it when SD was being enjoyed and didn't want that talent roster to continue on SD to win over the crowd, which is just so mind blowing to me.
Today both shows are struggling but I have to admit that Raw was the first show and WWE management seem to consider it their A show, even though SD clearly proved otherwise most of the time.