It is impossible to pick just one. I am a JMB fanboy so take this with a grain of salt. I am also only listing guns I own not guns I want or have shot once or twice.
1911 "the Worlds Finest Close Quarters Sidearm… and...King of feedway stoppages.” -Ken Hackathorn There is nothing like a 1911 trigger. I have smaller hands so double stack 45s don't work for me but the slim profile of the 1911 is perfect. This is a stock 70 Series Colt that is my baseline for the platform. It is stock except for the grips and while it does not have all the bells and whistles of some of my other 1911s this particular gun just shoots. It is and amazing pistol right out of the box.
BHP this is by far my favorite semi-auto in 9mm. With a little custom work they are fantastic shooters and they fit my hand better than any other double stack pistol. It points naturally and like I said with a little trigger work, good sights and good thumb safety they are great shooters. "Not as random or as clumsy as a (Glock); an elegant weapon for a more civilized age."
Sig P228 This is the gun I really cut my teeth on. It was the first serious high quality pistol I ever owned. I have moved away from DA/SA these days but I still own 3 of these. They are perfectly sized guns which balance well for me, even if they are a little top heavy with their high bore axis. I love the P220, P225 & P226 as well but the P228 is like the Goldilocks of Sigs for me. Its just right.
From there the waters get muddy. There is the rabbit hole that is CZ 75 pattern pistols. 75B, Shadow 2, Sphinx SDP, CZ P01 etc... they are all solid shooters and have their place in the safe but do not trump the 3 above. Glocks get an honorable mention but they are like driving a Honda Accord vs a BMW 3 series. Dependable but kind of boring and lack a soul. I have never been a HK guy. I find their DA triggers to be some of the worst out of the box triggers I have ever encountered. One would think at that price point they could do better. I do like the engineering behind the P7 HKs but I don't really enjoy shooting them. The good thing is there are amazing choices of all kinds these days to match any need or want at almost any price point.