Below are the most important posters I've got up in front of my classroom. I put these on heavy colored posterboard and write nice and big, and refer to them quite often in the first months of class. These are one of administrators' favorite things to see visibly displayed for students, and they are an easy way to colorize our often stale environs.
This Rules poster is newly formatted, a combination of Ben Slavic's and Rita Barrett's. I received some helpful feedback from a rock star Spanish 2 class about it. They think the new poster is better.
This Commanding Comprehension poster is indispensible. Students are still referring to it in December, though many know them by heart now. With this poster, there is little excuse for not comprehending. Also, each phrase on this poster has an accompanying gesture in case they are uncomforable verbalizing them.
Question Word posters... es obvio. When I make these, I use the fat Sharpie for the Spanish so that it appears bolder than the English. (oops, Can you find the missing accent mark?!)
There are other posters in my room, but these are the big dogs that bite hardest.