my string frays really easily and i thought it was time for new string.
YoYo StringLab. I use the Type X, Its really smooth and soft feeling last a good amount of time and its sold here.
I can’t get enough of Twisted Stringz. I know some people really don’t like them, but I adore their Kevlar mix. And they last such an incredibly long time!
Andre carries quite a varied selection of the more established brands here: http://shop.yoyoexpert.com/category/3/Yo-Yo-String
I suggest starting with 2 or 3 different choices to see what you like, or don’t, about each to help narrow down your favourites. Much like yoyos, string is ALL about preference.
You will likely end up with more than 1 “favourite” as time goes on.
I hope this advice helps you on your journey.
/paul
My favourite would be Kitty polyester strings in either normal or fat. YYSL Type X is also great but a bit pricier because it isn’t sold in large bundles.
Yuki
I like 100% poly strings from Yocala off eBay. But that’s just me.
Big Yoyo string Type 3 or Slix loose are my favorites.
listen to him, he’s the grandfather of string. 
I would highly recommend you try out expert strings, kitty strings, and standard poly. And even try making your own string.
I recommend something poly strings, its on par with many premium strings, but much cheaper and sold in 100 bulk. My choice for contests.
I reccomend Big Yoyo String. Why? Because I make it of course! Haha, but also check out the offerings from Toxic, YYSL, and all those other good companies. There has to be something out there that fits your fancy
Nylon dead thread i have one string that has lasted a month and is still holding tension great, along wit whipping like its brand new