I can think of a few good signs:
1. Communist: 100 million subjects murdered and counting
2. Just because an idea has never worked, doesn't mean we can't sweap away the corpses and try again.
3. Whats the difference between communists and Nazis? At least 12 times as many people were killed by the Communists.
4. Communism: Genocide isn't bad if its done by your own people. Just ask the Khmer Rouge.
5. Imagine a world without kiddie shills for mass murderers.
6. (With a picture of Lenin, Stalin, Mao, Pol Pot, and Castro, next to an approximation of the those they killed) "You Can't make an omlette without breaking a Few eggs." Alternate text would be " Being a communist means never having to say sorry."
7. "When the revolution comes, will you accept a credit card for my executioner's bullet?"
8. "What is Ingsoc for Gulag or Logai"
9.
Nazis: Arbeit Macht Frei (Work makes one free)
Communists: Re-education through work
Capitalists: Work makes you rich
Which one are you marching for?
10. If communism Works, why do communist countries always need walls to keep subjects in?
11. If Capitalists are such war mongers, why is the only decent thing built by communists an AK-47?