The one that fits your hand
For my money the above coupled with how it actually holds on target while I shoot it. Also applies to the one I purchased for my wife who's hand is nowhere near as large as mine.
That said, I have shot plenty of different pistols in 9mm. Nothing against most any of them, they were plenty accurate and never missed a lick feeding or firing. Now some of them did not feel right in my hand, and I had to adjust things after each shot to realign the sights on target. Granted practice will eventually alleviate that issue, but how long will that take and what will it do to other pistols I already shoot.
I have shot something 1911 for most of my life so that simply fits and feels good, and I seem to be able to align them right on target shot after shot. Throw a Glock into the mix and while I CAN shoot them very accurately they simply do not feel right, and things do not align for me naturally. Not dissing Glocks but they just do not appeal to me.
I shot several different 9mm pistols before deciding to purchase my P226. I like the way it felt in my hand, and when I raised it the sights seemed to align right where they should. I love the short stroke trigger which gives you immediate recovery fire ability. I also like the uncocking lever as well.
For the wife and her small hands, we simply went "shoe shopping" trying on everything we could get her little hands on. She was the one who liked the P320 Compact hands down because as SHE put it, it fits my hand real nice and it is light enough for me to hold comfortably. I personally have almost two fingers hanging off the bottom of the grip, but it still feels great in my paw as well. I'm not overly wild about striker fire and not here to debate the issues of one over the other. I also like the option to change out frames and calibers within reason for a little less than another whole pistol would run, only not having to deal with the paper work that follows it around.
To be honest there are plenty of "personal" reasons "WHY" one might be more liked than another. The bottom line for me is how well, and how many times you can hit what your aiming at in a hurry. It don't matter one whit which one you have if you can't hit what you need to, when you need too.