I would say catcher too, but in the name of moving the convo forward I am gonna go out on a limb and say first base is the next most important position.
Lots of teams have good outfielders, shortstops and other infield guys, but if your first sacker cant glove the ball its a bad sign for pennant hopes!
First base defense is under-appreciated, and alot of the best plays go unnoticed or expected. First basemen aren't always the most athletic players, but they are usually the best at putting the ball into the glove, which is the essence of defense.