for me it has to be passing. if you can pass well you can make the others easier, with the exception of tackling. you need to tackle well to get possession, and then once in possession if you can pass well you don't need to dribble as much and you can make shooting opportunites easier.
it's often said that dribbling is innate and is not a skill that is easily learnt. however, if you are lucky enough to have this skill you can turn a game on it's head. passing is more of a team skill, whereas the others are individual. it really depends on your personality, the situation and the ability of the team as a whole.
i love to dribble and shoot, but at the end of the day if it's a close situation there's nothing more entertaining and motivating than playing in a team that can out-pass their opposition.