best mmo is highly subjective, lol. it wasn't even the best mmo when it first came out, it was just very polished and did things right.
but, it's time has long since passed.
Imho, WoW won't die tomorrow, or in the next 5 or so years for that matter. I'm no fanboi (I've played it on a off for 7 years, finally quit the game because I did get tired of it).
It is ONE of the best P2P MMORPGs you cannot deny that. Denying that would be foolish. I'm not saying it is THE best, but one of them.
It has surpassed all of the "WoWkillers" without taking much of a hit subscriber wise. They are losing speed I'll give you that, but they still have enough momentum to continue on. And I'm pretty sure they are still making more than enough money to covers their expenses.
I would even bet on the fact that they are probably making more money than what they are spending and that the game is in NET PROFIT.
Eve Online is also very good but it serves a different playerbase, same as DarkFall.
Wildstar and TESO are the next "WoWkillers" and I highly doubt they will make much of a dent in their numbers.
I don't think it has anything to do with the games themselves. I think it has more to do with games like DAoC / WoW / EQ (in the prime days) felt so polished and well thought out (to some extent) that paying for any MMORPG that came out as P2P (Looking at you TERA / SWTOR / LOTRO / AION / etc) without the same amount of commitment and polish was wrong. Players felt cheated to pay monthly for some "lesser game that isn't X" and would much rather continue to play X game even with all it's flaws.
The market has evolved, people play F2P games now. P2P is a thing of the past. And once Eve and WoW either go F2P or close down, P2P games will no longer exist, and no game company will make one.