Best is meaningless.
It is a great MMO with really good content but end game and leveling. PvP is good. It also has a lot of very nice systems like group finders and the like.
But if you want a MMO set in the modern age with lots of puzzles then WoW sucks and The Secret World is better. Or if you want to be a Jedi then SW:TOR is a much better option.
But if you want a fantasy based game with good end game content you'll find it hard to beat WoW