I love using the Metro (or subway, Underground or however you wanna call it) in every city I visit. It’s cheap, fast, easy to navigate, great for people-watching and here and there you can also spot some great artworks. In Paris I loved the Arts & Metiers Station on Metro Line 11. The whole station is an artwork, honoring the museum of the same name that is located above.
More great metro stations and art underground in New York, LA, Barcelona, Lisbon and Berlin can be found here.