Walter Mashburn just seems so jovial and charming all the time, someone who could be friends with Patrick. So that makes him the least obvious, and in my book that means he's Red John! :-P

Also this was bugging me...In The Red Tattoo episode, after Patrick outs Kira, Cho asks her, "Who hired you? Was it Visualize?" She just says "Uh yeah." She didn't out Visualize as the group that hired her, she just agreed with what Cho said. Later, when she answered her door, she said, "Oh Hi" in a way that showed she already knew the person. So when Patrick arrived at the crime scene and asked her "who did this to you?" why did she give him a description, that is, the tattoo? She could have just told him a name. The killer left her alive and told her to only tell Patrick that "...he had a tattoo on his left arm" while depicting it in her own blood. It was Red John who used the PI, not to gain intel at the CBI, but as part of his cat and mouse game with Patrick -- so he could give him the new tattoo clue.