The GitHub graphql API is pretty neat, mostly because it's a great example of a product where graphql has advantages over REST. As a code reviewer for repos with hundreds of simultaneous PRs, I use it to filter through branches for stuff that needs my attention the most.

NewRelic's NRQL API is also quite nice, as it provides an unusual but very direct interface into the underlying application metrics.

I'm also a big fan of launchlibrary, purely because I love spaceflight, and their API is an extremely rich and actively maintained resource. This makes it a great data source for playing around with plotting & statistics libraries — when I'm learning new languages or tools, I prefer to make something "real" rather than following a tutorial, and I often use launchlibrary as a fun and useful data backend.