Nothing else even comes close for deadening flats. Eucerin Aquaphor does OK at it, but it's only OK. And forget the other medicated formulas. Plain white Eucerin cream is the ticket for making your new flats thumpy quickly. Store brand substitutes are fine as well, but some can be runnier than others.
Takes a couple overnight applications and wipedowns to really work, but the first application takes care of the worst of the zing while the second makes them sound like they've been played in by a sweaty person for a month or two. Even works on zingy-when-new Chromes. Results were more dramatic with Dunlop flats, which are less zingy when new, but the Chromes G string can be tricky. Still it's way tamed down and barely clinging to life. It also takes a decent amount of edge off new strings if you're pressed for time and can't do anything but one quick coat.
Best of all, you don't have to skeeve it like rancid chicken grease or some of the really nasty bodily secretions some of you suggest (you really need help, people). No need to turn your bass into a breeding ground for microscopic parasites if you don't have to.
You can argue with me till you're blue in the face about this or that foreign substance, but to put it simply, you are wrong. For deadening strings, Eucerin is king. If you use anything else, you're jackin'.
EDIT: This quick string deadening method does not work on TI Jazz Flats, any kinds of strings with similar silk or synthetic windings if there are any, or coated strings. The cream can't work its way into the windings on these types of strings.