I find that a standard 3 X 9 X 40 scope will serve me well out pst 3oo yards. My longest measured shot was 305 yards and that was made with a 2 x 7 X 35 at 5X. In my neck of the woods 300 yards is a long way!

You need a low magnification (4x or lower - lower is better) for deer hunting so you have a large enough field of view to acquire them quickly in the trees or if they are running. You can go with as large a magnification as you can carry on the top end and feel free to dial in on hairs at 1,000 yards if you want to. Deer are big animals and you can put that cross hair right where you want it at 600 yards with a 9x magnification. I use a 3.5-10x variable for most deer hunting and no deer I want will ever walk away because of a scope issue out to 700 yards. I keep it on 3.5x almost all the time unless they are posing way out there. I use a 5-20x on my 1,000 yard deer rifle and I prefer that high magnification at ranges of 500 to 1,000 yards but I won't carry in the trees or where I might be jump shooting them. The good old 3-9x has been popular for nearly 50 years for a very good reason. The only thing changing it is now variable scopes can go 4x magnification from the low end to the high end (e.g. 4-16x) without losing accuracy. Flexibility for more magnification is certainly better on the high end but really not often needed on deer.

I always like to make the most accurate shot possible. I practice using a dime-sized bull at a bit over 100 yds. I will dial my scope up. I use a much larger target with open sights, but that's because of my older eyes now.

So I use a higher magnification at shorter ranges. Where I'm at, there is little if any chance at a 300+ yard shot anyway.

3-9x40 seems to be most common and works well for me in most all situations-since 400 yds is a pretty long shot for my huntin grounds, I really have no need for any more magnification.

I have a 3x-9x and wish that I had gone for the 4-12. Just an option of being able to have more magnification for not that much more of a cost. It also can depend of if you are hunting whitetail or mule deer. If I bought a 4-12 I would typically leave it at 9x but I know there have been situations that I would really have liked a 12x capability.

I think low power is far more valuable than high power.If I bought a variable I would probably get a 2-7 not a 3-9.

For a long shot you'll have time to adjust your range but for a short shot you need the field of view that you won't have on 12X.

That happened to me 2 years ago. I forgot my scope was on 9X and walked up on a huge buck lying down in a small bush about 20 yards away from me. My scope was frozen, I couldn't move it and a beautiful buck got away because I could not find him in my scope.

ALWAYS WALK WITH YOUR SCOPE ON LOW POWER!!!

I really enjoy my 4x though.

My deer rifles are equipped with a 2-3/4X Redfield, a Weaver 3X, a Leupold 4X, and an aperture (receiver) sight. Briefly, I have no high-magnification glassware on the deer rifles. I expect to fire my deer rifles from the standing offhand position, and I do my best work under these circumstances with low magnifications, and a wide field of view. I should add, three of these (deer) rifles are carbines.
My varmint rifles are equipped with 7.5X, 10X, 12X and 16X glassware. These rifles are normally fired from rest at small, distant targets.
I have no disagreement with those who make other choices or express other preferences; I simply know what I prefer and what works for me.

I have a 3-9x40 on my 30-06, and keep it dialed low, and I have yet to shoot a deer with it. I hunt in forested mountains of PA, so there is a real limit on visibility. I doubt I will ever be presented with a shot over 150 yards, and the longest I've taken while hunting was probably about 80. That was with open sights on a Marlin lever-action. About two thirds of my deer were taken with 20 guage slugs with just a brass bead on the barrel. I want to see what's AROUND the deer, too, to make sure the shot is safe.
The sights, the action, and the chambering should all match the terrain. Open country (not like mine) requires high-velocity ammo, a heavy rifle, and higher magnification, if you're shooting from a rest. If you're moving in close on foot, forget the extra bulk and magnification.