To create the best poster, Photoshop is quite an essential tool to use for the main bulk of the image. You can create visually stunning high impact artwork, big bold headlines, and instant visual identity.
One trick I like to always test with poster design is to shrink the image you are working on to something tiny on the screen. If it looks interesting or can convey a message when tiny, it is doing its job effectively.
Below are some examples of my own poster designs.