Glocks are great in a good holster that covers the trigger in my opinion. The best concealed caliber of course is always up to debate. I like 9mm in that it gives you a very good size to capacity ratio and with top end high quality loadings is quite effective and manageable in recoil. The 40 and 45 will give you a slight edge in barrier performance and expansion, but that slight edge doesn't change my choice in 9mm for it's advantage of contollability and capacity. Lets say your choice may be a Glock 19 15+1 in the pistol and two extra mags gives you 46 rounds or you could opt for G-17 mags that hold 17 or even higher capacity mags and you should have enough firepower to hold down the fort until back up arrives. Or you could go w/ a G-26 and carry two G-19 or G17 mags for backup. 40 and 45 of course is very good as well, so I don't believe there is a best, just one that you shoot the best and feel most comfortable with. I myself carry either a PX4 subcompact or compact in 9mm as I prefer a good DA/SA for CC as it would be highly unlikely they would go off w/o some sort of extreme user induced negligence.