Khan Academy great resource for learning art history. It has great collection of articles, video lectures, and quizzes. It features topics across the world and time periods. It is obviously managed by knowledgable curators and mixes various media to communicate knowledge. It is very accessible.
Khan Academy only focuses on "traditional" art history, it does not include design or contemporary art. It also seems more like a study guide for the AP Art History exam rather than a resource for inspiration.
Google Arts and Culture has many high-resolution photos and a lot of engaging content (such as games meant to involve users with fine art with the integration of technology). It has a great search function with useful filters.
GAC has, at times, a confusing array of content. It can be hard for new users to navigate. It is also hard to find specific art pieces. In my experience, the site layout and UX changes all the time.
Both the MOMA site and app have great organization and navigation. It has a great search feature with useful filters as well as it has audio lectures/tours for visitors who are visiting MOMA's physical location.
The app and website only focuses only on MOMA's collection; It is more a complimentary product to MOMA's physical location and collection than a resource to learn about art history.