World of Warcraft
Since November, World of Warcraft has signed up more than four million subscribers worldwide, making for an annual revenue stream of more than $700 million. About a million of those subscribers are in the United States (with more than half a million copies sold this year) and another 1.5 million are in China, where the game was introduced just three months ago. By contrast, EverQuest II now has between 450,000 and 500,000 subscribers worldwide, with about 80 percent in the United States.
Ref. https://www.nytimes.com/2005/09/06/arts/design/06worl.html
I refuse to play this game. I have lost three friends to WOW addiction. Because of the way its set up, the game play experience is out of this world. I don't play it allot, only from time to time when I go over to help one of my friends out, but I play for a couple hours at a time. Personally, I think the head to head was a detriment to the game. Lots of people like it, and for a while there was the special custom armor prize, but when there are tons of people fighting, it slows down the server I'm on when I get to that spot. For content, nothing out there beats WOW. Even Everquest 2 isn't nearly as popular, which was really surprising.
Addictive is perhaps a light term, I am coming off it, experiencing the rpg equivalent of going cold turkey.
What makes it so good is its simplicity its easy to get into, easy to chat, easy to group and easy to quest. Its a pity its not easy to put down.
Add to this a strong setting, involving challange's and beautiful world its not surprise its the one to play
A lot of other MMORPGs suffer from sensual overload and weak interfaces best example star wars galaxy. So many random options and unfocused lvl progression mean fail to keep your attention.