Thats not really down to WoW though as such, its more down to what a persons first MMO was. Most people rarely find anything to compare to their first MMO and for a LOT of people, WoW was their first MMO. Hell there are still hundreds of thousands of people out there who actually believe that WoW WAS the first MMO.
For me , UO was my first MMO, and it is still to this day the MMO which I spent the longest in, I played it for just under 5 years (though UO itself has been going now for almost 15 years). No MMO since UO has lasted me as long, and no MMO since UO has compared to it for me. For example, I enjoyed WoW while I played it, it was colourful and fun, but all the while I played it I just couldnt avoid the nagging in my head that its gameplay was just not what MMOs should be about, that they should be more about the gameplay in UO.
Ultimately, I enjoyed WoW for a while as I played, but ultimately it was also a hollow MMO experience for me, it was too limited, too restrictive and lacked 90% of the freedom that existed in UO to be, and do, almost whatever you wanted. However it was a good game, polished (as to be expected with Blizzard) and entertaining, just not what I considered to be a "good" MMO.
However it has to be said that the kind of player who played those early MMOs like UO was a much different demographic to the kind of player who played WoW and many of the later MMOs. The different era of player had different aims, different goals and different wants from their MMOs. Part of why I left WoW was because I was increasingly feeling out of place amongst the players I was encountering and socialising with ingame, they just were a different type of player and put importance on different things to what I looked for in an MMO. I could sit for hours and try explaining why UO was so good, but I fear that many of the newer MMO'ers simply wouldnt "get it" or would find it hard to see why what I was saying was so much fun. Like many things in the past, its something you simply had to be there to experience in order to understand (bit like Daoc too)
All of that is why I say, for some people WoW is categorically the best MMO, for other people Daoc is categorically the best MMO, or Lotro, or whatever. Horses, courses, etc.