World of Warcraft! Boo yah! Im a level 80 death night and pwn. Anyone who doesnt believe me should download the free trial at worldofwarcraft.com
What is said above is an extremely biased answer. In terms of franchises the major hitters are Halo, Call of Duty, and Mario. Teens to adults normally stick with Halo or Call of Duty. Children normally play Mario. I'm not sure but I don't think World of Warcraft has nearly as many people playing as Halo, Call of duty, and Mario. in terms of sales Mario has the best sales(But has also been around longer and has more games). Halo and Call of Duty are roughly tied at over more than 25,000,000 sales a piece.
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Now I think it's Mario because you can go to just about anyone and ask them who he his and they will always respond "He's that short Italian plumber with the red hat." But then it might be a PS2 game because that sold 150 million units. I'm honestly not sure but its sure as hell not Club Penguin and Roblox.