Pocket carry is usually pretty viable : many sheaths come with a clip on the side that will hold the sheath securely in place. Not the fastest to draw, but fairly natural and not as questionable as most other carry locations. In most situations, inside waistband at either the 2:00 or 5:00 position, is fairly viable. A belt and a loose shirt will keep it covered, and the draw is both fast and natural.
You may also be able to do well with horizontal belt carry, but doing so is usually difficult to conceal for all but the shortest knife. Draw is fast and natural from both sides, though.
I'm personally against neck carry. It's impossible to keep both concealed and keep easy access, and is very limiting as to what size of knife you can carry, with a very unnatural draw. Similar problems to 'boot' or shoe carry : you have to either mess with your pants/dress to get to the blade, or show it off to the world.
The majority of situations where boot or neck carry is viable, I'm a bit fan of upper arm carry. Very natural draw, fairly simple to jury-rig, and remains concealed pretty reliably. A few long boot knives don't fit short arms for this, though, and heavy clothing gets in the way.
In more formal wear, inside shirt pocket seems to be the most assuredly concealed, but can feel heavy or leave the knife behind. Depending on clothing, you may
As to what knife... what are you looking for, and how much are you willing to spend? Utility? Self-defense? Quick Access? A Camp Knife (emphasis on sturdy material and survival uses)? A Plug Knife (cheap, light, and designed for control for uses that may be destructive)?