My black hunter long bow should be here this week and I've had some people say it's worth replacing the string, but I don't know what to get. I'm looking for recommendations! Thanks.

I personally would wait until I received it and shoot it with the string that is on it.  If you don't like it, then maybe put a new string on.  I couldn't find anything wrong with the strings that came on either my longbow or recurve.  I have never liked the recurve so put a d97 string on it just to see what difference it would make and honestly didn't see any difference.

I didn't try the stock string and can't make a comment.

I did build a 452x flemish string for my bh longbow and it works well for me. 

I appreciate the replies. I'll have to look into some of the differences in strings.

We got one for the archery club at school and the string that came with it seems decent enough. It's nothing special, but it's not junk like some other stock bows come with. Some string silencers and it was good to go!

Here is a link to a Tradlab article on silencers but it also touches on string materials.  I think the biggest complaint about dacron is the stretching, then the loss of speed.
https://www.thetradlab.com/silencerstudy

You're going to need a backup string eventually anyway, so why not break the one in that comes with the bow and get it all stretched out with nocks in the right place, and plan on eventually using it for a backup.  There are several fine string makers who are sponsors of this board, any of whom could make a good string for you.  I have had good luck with SBD (Silent but Deadly) strings, who I believe is still a sponsor.

I got a SBD string for mine shoots great. The string that came with it shoots well also.

Randy