Lovely thread. Looking back, I have fond memories of the Museum of Mankind, in a fancy building somewhere near Piccadilly. Not the biggest place, but if you were in the area, the carefully curated anthropological stuff was worth a pop in. Now absorbed into the (much less attractive) British Museum. Other dead museums: the Commonwealth Institute. I loved that place. By the time I was able to visit in the early 90s, I think it was past its heyday, but the variety an detail kept you absorbed. Also a fabulous building, now hosting a not so good Design Museum (tube map a classic? - you don't say!) But big advantage of the Design Museum is at least one very restful sitting area that few people use. Sorry OP, to choose from your list, I'd say upper floors of the science museum.