FF6 is a hugely successful RPG in its own right, with about 3.5 million copies shipped. It's got fantastic character, a great story, and good multiple choice scenarios. EGM gave it Game of the Month over Mortal Kombat II and Earthworm Jim, and later won a bunch of RPG related awards. Nintendo Power (admittedly has a bias) called it the RPG of the decade, and was cited as a milestone in the FF series (not as much as FF7 would be, but no one knew what was coming at the time). Nintendo Power also later called it the 8th best Nintendo game of all time, and even suggested it might have been the best FF ever (though potentially this was bitterness over FF7 being on the PS). It was later named the 3rd best SNES game ever, only behind Link to the Past and Super Metroid. IGN later called it the 56th best game of all time, which might not seem high, but when you consider the sheer number of games put out there in history, that's big. It's accolades go on and on in this manner. It's pretty unanimous it's a hugely popular RPG, and personally, I find it an amazing RPG, honestly, I enjoyed it more than FF7. One of the best RPGs of all time, flat out, and highly influential in later classics like Wild Arms of Suikoden.
Now, World of Warcraft. I bitterly admit it is one of the most, if not the single most, popular games ever. I won't bother with all the accolades - it's got countless Editor's Choices, games of the month, top selling lists, so on, so forth. Anyone acquainted with video games knows this stuff. There's 10 million players worldwide, which is incredible, I must admit. If you want to call this in terms of selling more copies, or universal popularity, it's WoW. However, I choose quality gaming, and FF6 has it. It's got a better story, hands down - WoW's story is basically "travel here, kill 12 of these ,bring me x item, here, have some items" or "go into this dungeon, kill this fellow, have some items", whereas FF6 has a rich, involved story. The gameplay is better in FF6, in my opinion. I hate MMO gameplay, the concept of forced multiplayer (and the fact that the vast majority of people I am forced to interact with are morons, assholes, or usually both).
So basically, if you want a popular game, choose yourself WoW. If you want a good game, choose my vote, FF6.