Combat? If you mean military sidearm then I would vote Sig P226 in 9mm. It would of been the general issue pistol of the US military if Beretta hadn't bid a lower procurement price. Very reliable and robust design, excellent ergonomics, easy to disassemble and clean, and a great performer overall.
In the military a pistol is a backup weapon or is used by second line troops who's primary role is not front line combat. Thus, it is a defensive weapon and needs to be light and easy to operate and handle. The 9mm is a very flexible cartridge which combines the traits of high magazine capacity, light weight ammo, good penetration ability, managable recoil, and decent killing ability. It can also be configed to fire high velocity armor piercing rounds for use against armored personel. The 9mm will be the top military pistol cartridge until the next generation in weapon systems is invented. If the pistol becomes a general issue weapon for infantry, it will need to be even lighter. When you have to haul around 75lb of gear in addition to your weaponry, every ounce of weight makes a HUGE difference.
Now, if your pistol is intended to be used for more than just a last resort as it is in the military......you have several options. In this case, it might be a good idea to look at US law enforcement trends. Not that LE procurement of equipment is based on any degree of intelligence and logic, but after several decades of looking at trends and looking at agency adoption from coast to coast you can get a general idea of what is effective. You will see a HUGE variety of firearms and calibers throughout the law enforcement community. Glocks, Sigs, Berettas, H&Ks,.....just to name a few, and the most common calibers are 9mm, .357sig, .40S&W, and .45acp. All of these models of pistols and calibers are quite ideal for "combat" use. Pick a pistol that is highly reliable first, then pick one that you like the feel of and if you like the controls, then find out which pistol you shoot the best overall, then lastly pick the caliber.
Currently, I choose the Sig P220 in .45acp as my "ideal" combat pistol. Highly reliable design, great ergonomics, great accuracy,......just a very well thought out pistol. Single stacked magazines are also more inherently reliable and have a longer service life. The only downside is that you have to change magazines more often. However, if you stay proficient you can do this fluidly and in some cases it keeps you from burning up your ammo too quickly. On duty I keep two mags front and left, and two front and right on my duty belt. I can draw them easily left or right handed and I have a total of 41rds. It's unlikely that I will ever need that many rounds, but it focuses more on the ability for me to easily reach a fresh magazine left or right handed......especially if I only have one working arm.
. But the SIGarms P-226 9mmNATO is used by US Navy SEALs and many other hard use spec ops units so ***....
