I'm LE and have found that the method that best serves me is a low profile IWB holster with the shirt tucked over the butt of the pistol. I've carried up to a Glock 17 in that manner and as far as I know, never spotted. A G19/23 works well.
I am of slim to medium build. I try not to wear exceptionally tight clothing. For the last ten years, I've used an Alessi Talon clip IWB holster because the hard nylon clip is about the most inconspicuous that I've yet found. High Noon makes a couple of synthetic IWBs that are pretty slim and don't print a lot compared to most leather holsters. Some holster makers offer a "tuckable" holster, but they add extra leather bulk to the package, so they might print a little, depending on the person.
Prior to getting the Alessi, I tried a number of cheaper IWB clips, and usually had to do some "surgical" modifications to them to get them to fit me. I finally gave up and carried a G17 and 19, as well as revolvers IWB "Mexican" carry without a holster for a while.
I carry a small pistol on my ankle or in my boot on occasion, and suggest that if you're going to ankle carry, you should check to see how much your slacks ride up when you sit down.
Most of the time anymore, I carry my primary pistol in an OWB holster with my shirt untucked.
Some of the shirt-holsters such as manufactured by Kramer or 5.ll might fit the bill if maximum concealment is required. Some of the tactial clothing manufacturs also offer shirts and trousers that are designed for carrying firearms. I've tried some of them, but they didn't work well fo rme.
All in all, you have to take into consideration your build, the manner and style of clothing you're going to wear, and the size of the gun you're trying to conceal. Don't forget how you're going to carry spare ammo.