9mm. Why? It's a perfect balance between firepower and capacity. The 1911 in 45 is still my favorite all around gun/ammo but is impractical for me to carry concealed on a daily basis year-round.
I am trained to double tap center-mass on a threat and finish up with one to the head. This means 3 rounds in quick order for each threat. This is no matter what caliber I use, .22 to .50. To me the .45 must sacrifice capacity (with a gain in bulk/weight).
I have heard uneducated people say "you don't have to double tap with a .45"........I ALMOST feel sorry for those people. Almost. Education is power.
Above all, shot placement is critical and I train enough and have won enough matches to be confident in my abilities to hit vitals with speed and accuracy. A 9mm is more than up to the task. Study balistics on humans and human-sized animals you will find that a 9 is about on par with a .45. ("Handgun Stoping Power" and the "Strasbourg tests") Here is a quick snip:http://www.chuckhawks.com/handgun_power_chart.htm
There is no huge Earth-shattering advantage of a .40 or .45 over a 9mm+P when it comes to stopping humans. This statement will surely incite a riot, but I care not. I have done enough research over the last 20 years.
The end result is having a comparably effective gun with 15 rounds at the ready (G19) in a fairly slim, lightweight, reliable package.
I'm ready for all of the arguments and haters because I was one myself just a few years ago. I admit that I made fun of 9mm and Glocks. I thought that there was no other gun in the world that mattered besides a 1911 45. Fortunately I opened my mind and educated myself.
If someone does carry a 1911, thats great. What a wonderful defensive handgun with few drawbacks. I can't do it down here in the deep south in shorts and a t-shirt without it printing though.