I disagree with this comment.
WOW, is nowhere near the best MMO on the market, it was surpassed a long time ago in terms of gameplay. It literally looks like you playing a game from 10 years ago, which is almost correct in terms of the games age.
Let anybody new to MMO's play something like Guild Wars 2 for a day then WOW for day and I'd bet they'd choose Guild Wars 2.
I tried WOW for a month when I was playing Age of Conan, and I've got to be honest here with you - I thought it sucked, I did not like it one bit. Character Customisation is terrible, you literally feel like your playing the same toon as 100,000's of other players, which completely ruined the immersion for me right from start.
Where WOW succeeds is the amount of players it has and the amount of money it makes. It's by FAR the largest MMO, constantly being updated and is generally bug free... plus it will work on any piece of shit spec PC. So in that sense it is no1.
If your talking about overall success then WoW is the greatest,and you cant compare Age of Conan or Guild Wars since Wow came out during EQ 2 days.