Best is subjective. It's the most popular MMO in the West, although there must be MMO games with a larger player base in the Far East but popularity doesn't mean that it is indeed the best. If you like popular entertainment in other fields, (TV, books, films, etc), then give it a go. If you are a bit picky about your entertainment, then there are arguably better MMO's out there for someone with slightly different tastes.
Two MMO's that are worth looking into are Eve Online and Guild Wars 2. Eve is a player driven sc-fi space ship game with a monthly fee, but it has a free trial, whereas GW2 has a traditional fantasy setting, but no monthly fee.
There are also a number of good free to play MMO's out there, Lord of the Rings, Star Wars, etc, so you do have choices.
If you want informed recommendations from people, then you need to give them more details, such as:
1. Have you played a MMO before.
2. Do you intend to play the MMO with friends or just solo it.
3. What type of games do you enjoy? (top 10)
WOW has a bit of a marmite effect on many gamers, (people either seem to love it or hate it), and many gamers go a step further and either love or hate the entire MMO genre.
Me? I have played a few MMO games in my time, so I'm not a MMO hater, but the grind thing is an incredible mind numbing waste of time, but they are usually more about the social aspect than innovative / interesting game play. That said, the game itself has to be interesting enough to keep you playing, and different games interest people in different ways. Speaking personally, I have never been in the slightest bit tempted to play WOW, as I dislike the graphics, and regard it as the worst offender when it comes to pointless grind.
Regards - Mr P