Missionary is by far my favorite position to watch my wife get fucked. Lucky for me, it's also her favorite position, and rarely does any other position. She thinks it is because in her high school days, on her back in the backseat was the only way her partners fucked her. And trust me when I say she had "a lot" of partners. All her dates ended with straight missionary sex.
Missionary is my preference for 2 or 3 reasons. First of all, it gives me the best view of his cock pumping up into her. Second, With her on her back underneath him, I can imagine different scenarios, such as her being ******/***** or maybe teens on a first date. Third, their's something very erotic about looking down and only seeing her arms and legs sticking out from under him.