Good question! Right now, I would have to say my Ping G driver. I bought it used about 5 years ago, and have
driven the ball VERY well with it ever since. I reshafted it with a 45 gram Aldila last year and continued hitting fairways with it. At age 79, I'm anything but long off the tee, so thinking it was probably "old tech" this spring, I bought a new Tour Edge; but it can't compare, and I've gone back to the old Ping (I'll be offering the Tour Edge for sale as soon as I can take some pictures, BTW!).
I hit 13/14 fairways with it yesterday and that just reinforced how important that club is to my game.....