We aren't often impressed by advertisements on the Underground but the Ocado "periodic table" one at Oxford Circus at present is utterly brilliant. The tagline is: Ocado: groceries for every table.
Put together for the annual CBS competion to produce advertisements with more text on them than normal, it made the final shortlist of six from nearly 500 entries. Even more impressive was the fact that it was produced by Ocado's in house team, not a top advertisng agency.
Ocado handle on-line food ordering and delivery for the Waitrose supermarket chain and their idea of relating things you can get in the supermarket to an imagined periodic table was a stroke of true genius. Amazingly they didn't win, but they should have and they were voted the passengers' favourite.