I visit the British museum almost every time I go to London. You can always find new additions and collections but what I like most is the ancient Greek floor with the collection of Parthenon which is unique. Another impressive collection is the Egyptian and Assyrian. You need a full day to visit all the museum. There is a very good restaurant with nice food or coffee and sweets, where you can have a break before you see the rest of the collections. In my opinion, you must go once to see it all and then choose some collections that you liked more to focus on your next visit.