I don't have a go to submission when I am on top, as I'm usually just trying to maintain position. If I am in mount I pretty much always start with ezekial to get them to give an arm. From guard I am always looking for the triangle. These days I avoid guard at all costs, but if I am more likely to "pull a triangle" than pull guard. I easily land 10x more traingles then my second most popular sub. However, this is just because of my body type. I know people who are always looking for guillotines, americanas, etc. The most effective submission varies from person to person.