World of Warcraft is probably the most addictive. Personally, I have never played it, nor do I have any desire to. I believe that games, by their very definition, are supposed to have an end. There is supposed to be a winner.
There was a study using MRIs and a special catalyst that showed that the brain of a heroine addict going through withdraw showed the same physiological problems as a teenager going through withdraw for WOW. Kinda scary!