ZS series - which produced the best IQ?
I have a ZS3, a ZS6, and now a ZS19 [ZS20 without GPS].
I honestly don't see a dime's worth of difference in IQ, certainly not enough to be concerned about.
There is a legend that the ZS15 has better IQ because people think it uses the same sensor as the FZ150, but I've seen no proof that it's the same sensor - just one whose specs sound the same.
I don't have a ZS15, but from the forum, I see no noticeable difference.
I, personally, got the ZS19 because it's about 1/4" thinner than the ZS15, ZS3 & ZS6, but has quite a bit more zoom - 20x.
Being thinner, it's basically the same size as the ZS3,6, & 15 with my Clearviewer attached, and pocketability is important to me, as is the Clearviewer.
If ultimate thinness is not important to you, a ZS6 or 7 might be the best: these have adjustable levels of Contrast, Sharpening, Saturation, and Noise Removal, as well as a really useable "Panoramic Assist Mode as opposed to the lame "Sweep Panorama" of the newer ZS19/20 or ZS15.
WHEW! I hope that covers it!
'He who hesitates is not only lost - he's miles from the next Exit.'