For me it is driver easily. As a 2-3ish handicap, when I drive it well I find it hard almost to shoot too much over par... but if I drive it poorly, I can shoot in the 80's pretty easily. I putt pretty well, I don't make a ton of one putts, but I very rarely three putt. I don't chip great, but I usually hit a lot of greens (on or very close, close enough to putt often). So for me it is always about my driver, it can be the best or worst club in my bag from day to day. I hit 3 wood very well off the tee, and I do use that for basically any hole under 400 yards. But get into the 450 yard par 4's and that isn't really an option. Driver makes or breaks my rounds every time.