What is your favorite golf drill?

I wanted to see what people thought helped there game out the most. Just trying to get some ideas of what everyone thinks about this and what they think is the most important part of the swing.

Twice a week, I play a game I call four-ball.  Its nine holes at my club during dusk hours (nobody else on course usually, but plenty of light).  I walk the course for this drill (only carrying a 7 iron, my four wedges, some balls and my putter), so it doubles as good exercise.

You play four balls.  You walk to about the 140-143 mark on every hole to "tee off" (or whatever your stock 7 or 6 or 8 iron is).  I hit my 7 iron four times - one to go right of the green, one to go left of the green, one to land short of the green, one to land long of the green.  If a ball doesn't hit in the right spot (for example you slice your "left of green" shot or you try a bump and run to be short and rolls through) or on the green the ball is dead.

You then try to get up and down with your full routines - off-green shot, then putt.  Then move to the next ball.  Each up and down is a point.  You see how many you can get out of 36 possible points.  My personal record is 27, but that was an *insane day*, usually I'm around 18-22 and I have what I consider a really good short game (hitting 230 off the tee I have to have one).  This drill does some pretty cool things:

1.  You get really used to ignoring the flag w/the iron shot, which is a mental block I see in bad players.  They *always* aim at the flag, and even when they don't, they tend to compensate in the swing to hit at the flag.  This really, really helps you get used to "pick a target, hit at the target".

2,  You get practice getting up and down from every and all type of lie, which in my opinion is the most important golf skill to shooting low scores once you can make consistent contact.

3.  You get a TON of practice getting up and down *at your local course* which helps immeasurably when you play "real" rounds like a club championship or a tournament.  You get to know everything about every green after 4-5 rounds of this and you know how to play all the little short game trick shots on your course.  Its rare after you play this 8 times you have a shot you haven't seen a few times before.  This is awesomely, insanely important - the biggest short game mistakes come from deceleration due to uncertainty about the shot you need  /chose - this drill, by making you hit these shots over and over (72 times a week) removes doubt from your shot and club choice to a large degree and leads to an accelerating, confident, downward blow in your short game.

4.  Believe it or not, after you do this a bunch and get good at it, you get much more confidence on the tee.  Given a 370 yard par 4 (pretty standard on my course) I only have to hit it 210 to have my 140 shot where I am very comfortable.  This makes you want to hit the fairway, because you are used to it, and you feel like you can make par pretty reliably even if you don't have a pitching wedge in your hand off the tee.

So, a little long, but that is my favorite drill.  Every time I do it I learn something about my course or my short game.

EDIT: Just read the part in your post about "most important" part of the swing, so maybe this wasn't what you were looking for.

@ JC1982, maybe not what the OP was asking about, but I'd be super into trying your little game if I had a place to play where that was at all possible.  Great mental and short game practice.

To the OP, my favorite drill changes depending on what's troubling me, but right now my favorite is the 1/2-3/4 shot where you don't let your right heel come off the ground even through the follow through.  This helps me keep my hands in front of the ball and keep my spine angle cause it doesn't let me push my hips towards the ball, only forward.

Like mdl mentioned, my favorite drills change based on what I'm working on at the time.  Currently I'm trying to get my putts/gir down, so I'm working on my putting stroke.  I like to set up 2 tees even with the ball and about 1/4" from the heel and toe of my putter.  This really helps me keep my putter on plane and helps me strike the ball with the center of the putter face.  I also like to lay a tee down behind the ball to work on striking up with my putting stroke, trying to eliminate "skipping".  If you hit the tee, you're probably pushing the ball which will cause it to skid and skip before coming to a true roll.

I second mdl's drill above for iron play.  I'm constantly working that drill with my 6, 7 and 8 irons.

Exaggerating whatever I am trying to achieve.

If I know ( and I do know ) that I take the club too far inside too quickly, I will exaggerate my backswing over and over and then faster and faster to get the feel of brining it back more upright

It's a constant battle

I will then do the 9-3 o clock swings.