Which certificate is most useful for a software developer (full stack)?

Nothing, wherever you will be applying they will be looking for 3-5 years of experience and believe me getting your first job is very difficult.

Best of luck from my side as I still have not got my first job as well.

Software companies don't care about certificates. Perhaps, top-notch companies like Facebook, Google, Amazon, etc don't even look at your college degree or undergrad major.

Instead of focussing on certificates, it's better to concentrate on improving your skills. Learning new technologies, and implementing your own ideas or optimizing existing solutions. This demonstrates a willingness to learn & apply, and would help you in the long run. It will give a headstart to your creativity and help you in getting things done independently.

Also, never forget your fundamental Data Structures and Algorithm skills. These skills will be useful for every position that you apply.

Depends where you are really but in general an undergrad degree (any hard science will be considered transferable by any company that knows what they're doing) doesn't hurt. There are a few fast track programs which are well respected such as here in Edinburgh we have Codeclan (codeclan.com/).
Essentially bigger companies are going to want to see qualifications and likely degrees. Also experience. Smaller companies, especially startups will look at any body of work you present them, qualifications or not.

Companies have stopped caring about certificates (at least for Tech). They do not care about the certification that you have done, They don't even care about your college degree. All they want is a true talent, so my suggestions are

• Contribute in Open Source Projects on Github
• Contribute on StackOverflow and try to earn some Reputation Points and Badges by Answering or Asking questions.
• Share your knowledge in the form of a Personal Blog or on online platforms like Dev.to or Medium.
• You can showcase your work in form of some open code snippets on CodePen
• Participate in Discussion forums like XDA

Yup, that's all on the top of my mind right now.

Best.

I think more than certificates, showcasing your skills through a project is better. Shows what you are capable of and gives you more credibility.

Just as Raksha said, your own projects are the best certification you can get. Build something that solves a problem you have and show it to the world!