This is a really hard question to answer as so much of my string choice is circumstantially related to which instrument I am stringing, and where and how I will be playing it.
I tend to base my preferences on a combination of three criteria:
1. Playability- This has mostly to do with tension and gauge, how the strings feel under my fingers when I am playing, how hard I have to push to get clear notes. I tend to prefer lighter tension strings with a smaller gauge.
2. Sound- Clearly, what the instrument sounds like with the strings, but also how quickly they keep in tune. More specifically do they sound good when lightly finger picked and also when plugged in and hammering away with a pick.
3. Longevity- How long will these strings last being played daily under my admittedly heavy handed strumming, and will they break? I prefer my strings to last between three weeks and a month.
All these things considered:
Regarding my main player, which is a KALA mahogany tenor, if I am concerned with having the best sounding strings to play acoustic with a good loud sound and I want them to last as long as possible, I go with the GHS blacks. If I want the uke to sound great plugged in, but I am not as concerned with acoustic volume or durability my choice is the Aquila Nylgut (no wound.) The Aquilas, by the way, are also some of the softest strings around with the easiest playability in my opinion. I am also quite fond of the KALA reds for sound and playability but they probably score the lowest in longevity. I stay away from big, fat, super hard strings like D'addarios and other hard nylons like Ko'olau golds, mainly because they don't jive well with my playing style and beat the hell out of my left hand fingers.