There are a ton of websites that provide you with courses and certifications. It's very hard to tell which is the best but here are some websites that have high brand value. The brand value of these websites will increase the significance of the certificate that you earn. These high-brand value websites are:
Tutorialspoint: Every engineer is well aware of Tutorialspoint. If you had ever searched for any engineering concepts online, you would have most probably visited their page. They provide courses on their platform and have a huge collection of courses. The best part is that they also have various certifications that cover a specific topic in depth (From beginners to advanced). They also give training to corporates.
Coursera: Coursera is a platform which has a large number of courses. They provide courses in various fields. Coursera ties up with some colleges and provides us the certificates. They also have tied up with Google for providing certificates.
Udemy- Udemy is a platform that brings instructors and learners to a single platform. Learners will find a large collection of courses.
Simplilearn- Simplilearn provides course certificates in various fields in technical domains. They offer courses with college tie-ups too.
I would recommend you to go with Tutorialspoint or Udemy for courses. They have great brand value and affordable prices. But if you want to learn a course from absolute beginner level to advanced, then I would suggest Tutorialspoint. They have curated certifications for various in-demand topics.