There are a million different threads on that. In a nutshell pick the answer you agree with most from the following:
1. Its all personal opinion
2. Aquila
3. Worth (the brown ones look ugly)
4. D'addario
5. fishing line works fine
6. Buy a bulk pack of differing brands from MusicGuyMic on Ebay and try 'em all.
7. Its all dependent on your uke. If its mahogany, use D'addario; Koa, use Aquila.
Personally, I'm still not sure of the best brand of strings. I have ordered a set of Worths, a set of Aquila and a set of D'addario for my concert (mahogany). I plan on trying them all and staying with the ones that feel and sound best. I like the Aquilas that came with my laminate mahogany soprano, so I'll leave that one as-is.