What are your essential iPad apps?

I've just won an iPad mini LTE from work and I have no idea what to do with it. As of now, it's a $450 calendar. Please separate suggestions by their function. (Productivity, storage, gaming, etc.) Also, I'm a philosophy student, so feel free to suggest any niche apps. That being said, what are your essential apps? It's worth noting that I'm an android user as well.