well honestly, I'm using factory ammo... with that said, I'm looking for the most efficient and hardest hitting. (snip)
Your comment is very reasonable and is the typical response of most hunters. Unfortunately, even this is ... complicated.
The fastest kills are direct central nervous system hits with bullets that transfer energy effectively and are not deflected by bone. Similar reactions can occur when game is hit by high velocity bullets that transfer energy quickly when the circulatory system is in a certain state; this is the "hydrostatic shock" that people talk about. In both of these circumstances, the game will drop on the spot, or "DRT" as it's commonly described. Notice, however, that the bullet does very different work in each case. In the first situation, the bullet will impact CNS protecting bone, either immediately or after impacting muscle or other bones. In the second case, flesh or lungs are hit, possibly very light bone, like ribs. A solid or fmj bullet, can be very effective for the first shot, but is very unlikely to produce the desired effect for the second shot, which requires quick and violent energy transfer against only moderate resistance.
The most common bullet failure mode of the first shot is for the bullet to break up and fail to break bone. The bullet fragments and the pieces slide along the bone, causing an ugly wound but failing to inflict immediately fatal damage. The bullet breaks up and doesn't penetrate. The most common bullet failure mode of the second shot is for the bullet to either fail to expand or to expand only moderately. The lack of expansion reduces energy transfer and limits the diameter of the wound channel, thus reducing the hydrostatic shock to an insignificant level. The bullet punches a modest hole, but doesn't do much damage.
The next fastest kills are those that cause immediate blood loss and the resulting blood pressure drop. A non-DRT heart/lung shot is a good example. These hits cause enough damage that blood pressure drops within 20 seconds or so and the animal just falls over. The bullet in this case may hit bone, flesh or lungs or some combination of the three. The most common bullet failures in this case are lack of expansion and excessive penetration, excessive expansion and insufficient penetration and, occasionally, bullet deflection.
Everyone knows most of this, if only empirically, so what's the point?
Well, the point is that "efficient and hardest hitting" depends on both your target and your chosen point of aim. If you only take lung shots, you can use quick expanding varmint bullets most of the time and get excellent results. If you only aim at bone, you can use fmj bullets most of the time and get excellent results. Most game bullets are a compromise between expansion and penetration. Expansion transfers energy faster and causes large wound channels, but penetration lengthens the wound channel and may be a necessity to reach the vitals of larger animals. Complete penetration may aid in trailing your quarry, but some energy transfer is obviously wasted.
Most "normal" factory ammunition is a compromise. It is designed with a primary use in mind, which for most big game ammunition in the US is reliable performance on whitetail deer. It will expand reliably under most circumstances and it will penetrate reliably under most circumstances for most cartridges of "normal" velocity. Once you step outside of "normal", you have to pay more attention to get your expected results. Some examples are of abnormal situations are shooting an elk at 30 yards with a conventional 150 grain bullet out of a .300 Weatherby. The bullet is very likely to disintegrate on impact if it meets significant resistance, which may or may not be what you want and expect. Hitting a coyote at 800 yards with that same bullet will produce a very different effect on the bullet; it may not expand at all.
I think that by "most efficient and hardest hitting" we mean that we want the animal to drop DRT as often as possible. When that doesn't happen, we want a good hit to be followed by recovery within 50 yards or so. Again, for most normal cartridges and most normal situations, which ever factory ammo shoots well in your rifle is likely to do excellent work if your bullet weight and type is appropriate for your quarry. If you feel that you're likely to have an exceptionally high velocity at bullet impact, switch to a tougher or heavier bullet. If you feel that you're likely to have an exceptionally low velocity at bullet impact, switch to a bullet of lighter construction.
At the end of the day, we want a bullet to hit the target at a velocity for which it was designed and we want the penetration / expansion ratio to be as we expect, whether that be high penetration and low expansion, vice versa, or somewhere in the middle. Ideally, we'd like the trajectory to be as flat as possible, since that simplifies aiming greatly, so velocity and high ballistic coefficient values are very helpful. In counterpoint, it is important to remember that trajectory is meaningless at ranges under 100 yards where the vast majority of deer are shot.
I suppose the moral of the story is that only you can decide what is most efficient and hardest hitting, since the terms mean different things to different people.