Ideal pistol? Anything smaller than a full sized 1911 will work with the right holster.
The key is the right holster. You need a In the Waist Band (IWB) holster, but also one that allows your shirt to be tucked in. That slightly harder, but there are lots of good models which allow just that.
This is a holster I like:
http://www.usgalco.com/HolsterPG3.asp?P ... 0&GunID=28
Deep concealment, although it requires the Galco belt to function properly. It is IWB, allows the shirt to tuck in (there is a gap between the leather an the plastic attachment piece to allow this), and it doesn't leave any belt loops to indicate you are carrying a gun either. Unfortunately, they only make the plastic piece in black, so it will be more visible on light colored pants/belts.
There are lots of others too, like this one:
http://www.usgalco.com/HolsterPG3.asp?P ... 9&GunID=28
Don't forget there are lots of manufacturers too!