I am about to start a site in my niche of plastic products. It will include a forum, a blog and also some services & customers care pages. Should I create the blog in one of my sub-domain (for example: blog.xyz.com)or should create it the root domain (such as in the xyz.com), which one will be more favorable?
It does not matter if your blog is in root domain or sub-domain to get ranked well. If you do proper content marketing along with on-page and off-page optimization then both can rank good. But, as you had said that your site contains multiple scripts such forum, customer contact page etc, so its good to create the blog in a sub-domain. Create a fine homepage and link your all pages from it, it will give the site a proper navigation and you will look good too in the eyes of the search engines.
Creating a global homepage is good and clever option but blog in the homepage is also a good one too. A site's root domain gets the post links so it will be more easier to rank for your blog's keywords if it is in the root directory.
Although it doesn't matter in terms of SEO. But I think the blog should be in sub-domain.
I think in the root domain is better, as root domain always gets more links that means more tyour blog will get ranked and eventually you will more traffic along with conversions.
Root domain is the best, after ranking whole websites benefits to all sub pages.