There are so many good ones, it’s tough to rate the top posters.
After much careful thought, here are the top five, from my admittedly subjective viewpoint:
1) Uncle Sam Wants You. A simple, direct and effective recruiting poster.
2) Keep Calm and Carry On. Now far more famous than it was when originally printed. A classic.
3) Rosie the Riveter. Women are just as determined and capable as men to help in the war effort. The face of determination and can-do.
4) Che Guevara. Sure, he was a commie killer, but that stare! that hair! that beret!
5) Loose Lips Sink Ships. Ever so much more catchy than “don’t blab ’cause important info may get back to the enemy and be used against us in ways that hurt us”.