My vote for best all around strings for slap & pizz playing goes to the Innovation Silver Slaps. Hands down best strings I've played (so far

).
They have a fat thumpy sound, nice gutty slap, awesome feel, but have plenty of definition and just a touch of brightness. Their tension, to me, is perfect - nice and easy tension, but snaps back quick - not flubby at all.
They were staying in tune within a couple days, too.
Not too expensive, either. I love em. Best synthetic string out there for Rockabilly, IMO.