I I got certified as Product Owner yesterday. For a developer it might be unusual choice but I highly recommend it. Why? Well, I like to be more than a developer. To bring clients new ideas, unexpected value, something more (and yet not wasteful). To get there you have to think like your clients, like their users. You have to think about how you can help them. For me this certification was not that hard because I admire Scrum and I have this mindset for years. I just did not have the opportunity to work for anyone practicing (real) agile. Why not Scrum Alliance? That is easy. What they call "certification" is a quiz for kids and not a real exam worthy certification title. Exams from scrum.org are cheaper in a long term and cheaper immediately if you don't need to attend class. And they are yours for lifetime. No need to pay protection fees. If you pass this exam you will be proud at yourself.
Mainly APEX, sometimes SQL, PL/SQL, clean code and certifications