I have to agree with Dohle that KoAlohas really shine on the smaller scales. I've found some variation with their ukes (even the same models), but they've been consistently good, and the best sounding soprano I heard was a Silver Anniversary. I don't think it sounded better because it was a silver. It just sounded amazing and happened to be a silver.
My best sounding soprano is a semi-custom by Alvin Pops Okami. It's a variation of his UkeSA ukes, which you can find on his site, but I got mine before his site launched, and I don't know much customization he offers anymore. Ed Fiscella is on UU and can give you more information if you're interested.
I got a thinline Rebel soprano not too long ago. It sounds great, but not as good as the Hawaiian ukes. But something about the compactness of it makes it so much fun to play, more than I expected. I know some dislike the smallness of sopranos (especially the scale) but having a thin soprano is really fun, and these days it's the soprano I reach for most frequently.
I think there's definitely a spectrum between full and rich Hawaiian sopranos and mainland style sopranos with more bark. I lean towards the former. It's almost an East vs. West thing (not in competition but in style), with Hawaiian and Asian ukes more often producing what I think are more similar sounds, and mainland and European ukes producing another distinct sound. Maybe Hawaiian vs. Tin Pan Alley music? Koa vs. mahogany? Again broad strokes, it kind of falls apart under too much scrutiny.
I have a couple Martins, and had a Timms, and they sounded great, but I definitely prefer the KoAloha and Rebel, which are sort of cousins. I'd still like to try a Wunderkammer though, another attempt with barky sopranos.
I found Kanile'a the opposite of KoAloha in that they seem to suit larger sizes better. I think Kamaka have a pretty consistent signature sound regardless of size and are my favorite Hawaiian ukes after KoAloha. And there are tons of Hawaiian-made uke brands that are less well known, but can be found if you search around.