As poster research I have collected some very influential, memorable and high impact posters.
The poster below was part of a campaign from "stonewall", a gay, lesbian and bisexual charity. The phrase was so blunt and written in such a block font that it shocked people across the UK.
The poster below was created by Shepard Fairey in 2008 to promote Barack Obama in the elections. The colours are slight variations of the U.S. flag colours which symbolise patriotism. Obama is looking upward evoking a sense of "hope" in the future for America. The posters composition fits along with the Fibonacci spiral directing the eye along the word "hope" then to his circular logo, and then to his face. This implies that he is America's hope.