This is a great question - there's nothing like discovering an API that opens up new possibilities. I have worked with too many API's to list but one that will always have a special place in my heart is the @Twilio API. It was the first time I used programming to produce an effect I could see in the real world (e.g. write this line of code and you'll receive a phone call). It was also the API I used to create one of my first revenue-generating businesses.
I actually just saw Dear Evan Hansen two nights ago! It was amazing but I think that it would be lost as a film... I loved it but msotly because it was in person and it would not work without the music! The tech was also a large part of it and I feel like it would be lost if it were made two dimensional! How about you all?!