Undoubtedly, there has been no game in the history of video games 6 years after it's launch, which continues to sell and gain subscribers like the World of Warcraft has. Just to give a perspective here are some numbers:
1. 6.5 million subscribers at any given time. If you add in the trials then the number is well up to 10 to 12 million playing.
Source: http://www.bloggy.co.uk/rants/is-world-of-warcraft-lying-about-its-11-5-million-subscribers/
The bad news is 70% of trial players quit before level 10, leaving 30% to become regulars. This is understandable as WoW like every other game is not for everyone, but WoW running on a vastly outdated graphics engine still manages to have 6 to 6.5 million subscribers 6 years after it's launch.
Source: http://ve3d.ign.com/articles/news/52996/World-Of-Warcraft-70-Of-Trial-Players-Give-Up-Before-Level-Ten
2. Each World of Warcraft Expansion Pack have been the best selling games of their launch years among other accolades
Released in November 2004, the original World of Warcraft was the best selling PC game of 2005 and 2006. The first expansion pack, The Burning Crusade, was the best selling PC game of 2007. The second expansion pack, Wrath of the Lich King, was the best selling PC game of 2008.
Source: http://www.gamasutra.com/view/news/30845/World_Of_Warcraft_Reaches_12_Million_Subscribers_Worldwide.php
The latest expansion pack, Cataclysm, broke the PC gaming record for the best PC gaming sales launch selling 3.3 million copies in 24 hours. Previously, World of Warcraft: Wrath of the Lich King held the title with 2.8 million copies sold in it's first 24 hours. (See above source)
3. The World of Warcraft has no signs of slowing down
In 2006, the World of Warcraft had 6 million subscribers.
Source: http://pc.gamespy.com/pc/world-of-warcraft/731355p1.html
A year later, 2007, the World of Warcraft was up to 8 milion subscribers.
Source: http://www.ecommercetimes.com/story/55135.html?wlc=1293010869
In 2008, the subscriber base for the World of Warcraft was up to 10 million.
Source: http://wow.joystiq.com/2008/01/22/world-of-warcraft-hits-10-million-subscribers
6 years after the initial launch on November 23, 2004, with Cataclysm released the World of Warcraft had reached above 12 million subscibers.
Source: http://www.product-reviews.net/2010/10/11/world-of-warcraft-subscribers-reach-12million-mark-cataclysm-anticipation/
Now, there have been great games over the 2000s, but none have done so well finanically and content wise as the World of Warcraft. Since it was first launched in November 2004, the World of Warcraft, at any given time, can count on 10 million people around the world to attest to the quality of the game. In a year, you can count on the World of Warcraft to have at least 6 to 6.5 million monthly subscribers playing.
Yes, other games have sold more in terms of sheer numbers and other games have introduced genre redefining features. However, none have sold as well consistently over a 6 year course with no apparent signs of sales or subscriptions tapering off. Neither has ruled their genre like the World of Warcraft has ruled the MMORPG genre. Competitors from Final Fantasy 11, Warhammer, Conan, and others have failed to gain and keep the traction that the World of Warcraft has maintained and increased since it's launch.
I have stated my argument. Feel free to disagree, but remember the numbers are staggering. Yes, titles such as Wii Fit, Black Ops, Modern Warfare 2, and New Super Mario Bros. Wii have sold more in sheer numbers, but their business model is a one and done deal. The World of Warcraft business model is one of the future where players pay a set monthly fee to play a game where new content is patched in every year effectively giving the game an infinite amount of content and time on the shelf so long as the subscriber base is at a level where it is profitable to continue making content for the game.
Please be cognizant of the differences between an MMORPG and a console game if you want to dispute numbers.
