A high functioning sociopath!!
Here is a list version of what I wrote- which contains a comparison of the character of Sherlock to the most common symptoms of sociopaths (the list of symptoms comes from the work of Dr. Robert Hare who has written quite a few books on psychopathy and sociopathy) to see how Sherlock measures up....
1.) Superficial charm, not shy at all, ability to say anything they please, never tongue tied. - This is definantly Sherlock, he is never at a loss for words and says whatever he wants no matter how horrible or socially unacceptable. The only exception is when he meets Irene Adler.
"Yes, skipping to the part where your father was violently murdered..."
2.) Grandiose sense of self worth, view themselves as superior human beings- Sherlock thinks he is the smartest person in the world and quite often will refer to humanity as a whole, as though he is not a part if it.
"I knew the affect it had had on a superior mind so I needed to try it on an average one"
3.) Need for stimulation and proneness to boredom- "Bored!!! *shoots the wall* "
4.) Pathological lying- hmmm Sherlock does lie, but only when he's working on a case. Otherwise he is brutally honest.
"Really? I thought it was a big improvement your mouths too small now"
5.) manipulative- Yes Sherlock often manipulates others to get his way. Like getting John to help him on cases and Molly to let him examine corpses.
6.) juvenile delinquency- can you imagine teenage Sherlock???
There are way more symptoms but It was 11:00pm and I had filled the page requirement. Sherlock does in fact fit the vast majority of the 20 symptoms of a sociopath. So the show does an excellent job of portraying the disorder. What I find funny is that some of the traits that do not fit Sherlock (criminal activity for the sake of criminal activity for example) are displayed by James Moriarty. Very intriguing. Here is a link to Dr. Hare's full list of symptoms if you feel compelled to study it further: