In design terms, the Tube is best known for its iconic map, designed by Harry Beck. But an equally noteworthy part of the Underground "look" is its typography, designed in 1916 by Edward Johnston. (Although that date might not be right; the LT Museum says 1913 for some reason.)
Just as the map was a confusing tangle before Beck got his hands on it, the Tube's signage was a mess. As different lines had been built and operated by different companies, they had all used their own "branding" and signage systems, so the entire look was very confusing.
Johnston's harmonisation of the system was so successful it's still in use today. And as this article from creativepro.com points out, it's the clearest in the world. (That piece also has some interesting facts about the NY Metro, and equally interesting story.)
Another great thing about the Tube.
Image taken from the London Transport Museum.