I don’t know much about string company’s, I know the basic stuff, type 6 and a cotton and stuff, I just dont know brands… I bought some string and didn’t like it at all, so I wanted to know what string yyf uses. I feel stupid asking this 
Its there own string its actually pretty good i like it it holds up well and is great for slacks
The green Type 6 poly here on YYE seems to feel just like it to me.
Contact Yoyofactory! They can help you out!
yyf string they sell it on yoyoexpert (Its not the best by far, Kitty string is)
All a matter of opinion. There is no definite “Best.”
What string did you bought? I’m pretty sure they use 100% polyester type 6 on all modern style yoyos. You can buy from YYE: YoYoFactory String - 10 Pack – YoYoExpert
The string that comes with the begginer yoyos is cotton and also available on YYE. 
Really don’t have any clue about the brand they use, but if it’s available online, go for it!