''Don't ask me to pick just one,'' says singer Gladys Knight, when pressed to name her favorite soap. Since getting hooked about a decade ago, Knight was always one of All My Children's most vocal celeb fans, but these days, she's crooning a new tune. ''Now, I watch all the soaps really,'' she insists. ''I could never single one out. I know them all. I mean, I don't see them every day, but I do scan them.'' Knight's upcoming gigs include not only Pip-less solo engagements in Vegas and other cities, but also a new acting role, as a sexy nightclub owner in Fox's cop show Uptown Undercover, premiering next fall. She's not blue about missing more soap time: ''I like being busy thank you, Jesus! And I keep up with tidbits from all my friends.''