It kind of depends on what kind of sound you like, what string tension you like, and what sounds and feels good to you on a particular uke.
So experiment!
I tend to like bright sounding strings with a fast and bold attack, and relatively low string tension - that means I tend to Aquila nylguts best on most ukes. I like how they feel, I like how they sound, and they enahnce my playing style, making my playing sound phatter.
Lots of folks love flourocarbon strings, like Worth Brown's (mellow sounding), Worth Clear's (brighter sounding), Fremont blackline (on the brighter side), Koaloha Clear's (bright sounding), but I don't care for them. I don't like the feel of their super thin gauge, and most importantly, when I play my ukes with them, the tone sounds thin to me and lacking in sustain.
I also tried Ko'olau gold's which I totally hated - sounded deader than a doornail to me, but I've heard other folks play with them and they sounded super mellow/cool. I just don't have the technique to bring out their qualities.
I have found that different strings can make a ukulele sound like a completely different instrument.
So everybody and every uke is different - I had a lot of fun spending $50 just to try out different strings on my ukes to see what I thought. I've gone back to my Aquila's in every case though.