Hmm, this is a good question. My dad joke answer is the putter because you'll probably use it on every hole. Statistically/analytically though, the answer is driver, as that's where most penalties usually come from. Then there's the cop out answer, which is probably the truest answer and that's all of them, because like @Ghostwedge said, if the operator is off with everything, then that's when the really bad rounds happens.
My best rounds have come from the irons being on. My tee game and putting are more of a strength, my approach and short game are passable, but bordering on weakness. If I were to be really critical, short game would be considered a weakness because I generally am not hitting it close enough to give me a good chance to 1 putt when I do miss the green.