I'm wondering what it is commonly considered to be of high value/importance amine the types of publications in applied linguistics/language teaching ? Is it journal article, conference paper, book chapter, books...? What would be the best choice for Phd students seeking a position in Academia ( post-doctoral/assistant professor)
I would rank publications in the following manner:
books (reasonable to make plans while a grad student, but might be too ambitious for most PhD students to actually pursue while they're also working on their dissertations)
peer-reviewed journal articles, which I think I'd rank almost alongside books if you're publishing in top-tier journals.
If you're only just starting out in your PhD program and haven't tried publishing anything yet, I don't think it's a bad idea to start with conference proceedings (if they come up) or a book review (journals or even through Linguist List), just to get your feet wet. However, as you advance in your PhD program, I would urge you to go up the ladder, and focus on publishing your work as journal articles.
I would also keep in mind that in any given category of publications, there's a lot of variation along the 'prestige' spectrum. For example, a chapter in an edited volume that's being published by university presses like Georgetown or Cambridge would probably be valued more than an article published in a virtually unknown journal.
The most important publication for you as a PHD student is the publication that will award you your title. Normally that is your doctoral thesis although it can also be paper or two. That depends on your university.
After you get your PHD title then the most valued publications are papers (in peer reviewed journals) because researchers/academics are measured by the number of papers and even required to publish certain amount a year (creating of course a wide net of vicious incentives to publish as much as possible regardless of importance or quality, thats why there are so many predatory journals pout there and a whole business of ghost writing... yeah, I expect the downvotes from this.)
Then the industry (although it depends which) value books (but a book chapter would be a good start and then a book once you have more works to your name) and media content a bit more (Tech for example would value your github stuff and working prototypes/projects more than they care about a paper done during your studies).
So,the answer to what would be best for a PHD student regarding types of publications is to first foucs in whatever publication is useful to finish your PHD according to your school, and secondary about the type of publication that would help you afterwards according to your personal plans to go into the industry or go into academia/research.
NOTE: There's a middle but gray ground regarding research outside of academia and in that case articles might be of consideration for your case.