what type of string is best for yo-yoing?
As a string maker, I can honestly say that it’s mostly preference.
Some strings are better than others, it’s true. But honestly, if you’re just starting, pick up a bulk 100 poly pack and just go from there.
Then, as you get better, pick up some other strings to see what fits your style.
Also as a string maker i can ditto what he said. Like for example my friend likes one of my strings and my brother likes another.
Pretty hard to go wrong with a bulk pack of Poly. EIther YYE or Kitty Normal will do you just fine.
On the other hand, I know myself to be an experimenter and someone who likes variety and options. So right from the time I decided I was half-serious about pursuing yoyo, I started buying 10-packs of many different kinds of strings. I’ve still only scratched the surface, but I’m getting a good feel for what I like and don’t like.
It’s not economical to buy so many 10-packs, so I’m not recommending it unless you’ve got a spare buck or two and have an inquisitive nature like me. 
But I’m glad I did it (and am doing it! The quest continues!)