The "Ultimate Combat Pistol" is going to be whatever firearm you are most comfortable with; bottom line. I own Beretta, Colt, Glock, HK, Ruger, SIG, & S&W pistols and love them all for different reasons. I believe someone made reference earlier to the proper tool for the proper job and think that is highly logical and the underlying reason why I own a variety of pistols. Having said that if I could only choose one pistol to serve as my combat pistol to defend myself and my family I believe I'd bring my SIG P226 MK25 and all of my Mec-Gar 18 round magazines. I'd take my SIG for a number of reasons but I believe the most important is that I shoot better with my SIG P226 MK25 than with any other pistol I own. 9mm is very manageable, 18 round magazines are generous, and I'm not military so I won't be shooting NATO ammunition. Many people don't fully understand ballistics or initial vs. secondary wound channels. I'd direct these people to view YouTube and have them watch some videos by Brass Fetcher. I'd be more than satisfied carrying a 9mm pistol with the correct ammunition.
Of all the pistols I own or have owned I will definitely be picking my SIG P226 MK25 as my ultimate combat pistol. I own a SIG P229 Elite Dark .40S&W as well and while I actually like that pistol a little more than my MK25 I am far more accurate with the MK25 and can hold more ammo.
The only other pistol I'd like to reference in this response is the Glock 17/19/34. I am also a fan of Glock and have owned quite a few of there pistols. Awhile back I decided to sell most of my Glocks to purchase SIG pistols as I had developed a preference for them and honestly was a little bored. I kept three of my Glocks and will not part with them, one a Glock 17, two a Glock 19, and three a Glock 20SF. I cannot shoot a Glock nearly as accurately as I can shoot a SIG, even with upgraded sights (Heinie, Warren Tactical, etc). I kept the two 9mm because of their amazing magazine capacity and interchangeability. I kept the 10mm because its a great gun, not many manufacturers make 10mm pistols, and there are so many aftermarket parts for Glock. One thing I find remarkable about this Glock pistols is there loose tolerances which pretty much allow them to operate through anything and the fact that they are so easy to clean. I'm a little OCD when it comes to cleaning so it takes me a good 30 min to 1hr to process a SIG pistol to my standards. I can process a Glock in less than a quarter of that time.
Best of luck and safe shooting!