I know it could be an opinion-based question between two groups(ISTQB vs BBST) but I would like to say as the content is invaluable for anyone who wants to be a better tester. if as freshers want to start his/her career then which one will be more valuable from the content & skill point of view. which can help most to the candidate in the competitive world or make someone a good test tester?
This might be an opinion-based question, so I'll add my opinion.
I've yet to find a job description that would mention BBST, but I've found several that mentioned ISTQB among the requirements. PDHide mentioned something similar in the comment section. You can try to search on some job portals and gather more information on this. I've also been asked some questions from the exams during interviews. However, it's important to know that frequency does not automatically equal quality or usefulness or whatever. It very well could be that ISTQB is more available, therefore it's easy to want candidates to have this exam as at least some sort of proof they are serious about testing.
You can actually see the syllabus for ISTQB online: https://www.istqb.org/downloads/syllabi/foundation-level-syllabus.html That's actually all that will be tested during the exam. I still think you can learn something from it, but it's important to balance it with some context and practice.
On the other hand, BBST seems to be a bit more practical:
"Buzzwords" such as domain testing, risk-based testing, bug advocacy, testing tours, context-driver testing are highly relevant, yet ISTQB doesn't seem to mention them. It's partly just a different terminology, but that matters as well after all.
Lastly, I don't think this should a either-or question. You can sign up both for ISTQB and BBST. They will bring different values to you, and you'll most likely learn something from both. If, however, you want to focus only on being successful "in your CV", ISTQB seems to have a bigger reach.