Best MMO of all-time for sure. I wouldn't judge my favorite game of all-time out from how much time I've invested in it though, different games are made to be enjoyed in different lengths. WoW is only truly good if you play it a lot, but something like Super Metroid can, imo, outdo it even if I beat the game in 3 hours. Earthbound for example is my favorite video game of all-time, it takes a couple of days to beat it for me, but those couple of days are simply magical. WoW takes weeks or even months to enjoy to its fullest, and even then it isn't as good. Still, I wouldn't have stuck around for 8 years if it wasn't in my top 10 games at least.
Now of course this is all subjective.