This is bar far and away my favourite museum in London, and even though I find it hard to put my finger on why that is, I will always come here first if I have a spare afternoon.
I think it’s the honeycomb effect of the building that forces you to properly explore if you wish to find a certain room (good luck finding drawing and paintings on your first trip if you have mapping issues like me).
Bonuses include a great little cafe, a warm building in the winter (trust me as an ex-British Museum worker this is important!) internal courtyard, bust room with plenty of seats to sketch from and an elaborate set up of rooms to showcase cultural treasures. Highlights for me are the costumes, metal working display, and the huge sculptural room, where on one side you can walk amongst the artefacts, and on the other you can watch the restorations in progress.
Come for an hour, loose a day.