Well not exactly necessary true, though true according to some. Which game or games much make up the MMO genre where it is right now? I will make my list, and then feel free to post yours:

- WoW: leaving WoW out would be like leaving out most of the MMO community. Some would argue that it would leave us with the true MMO gamers. Then probably an even better reason for picking WoW as an MMO defining game; - the one game to encompass what the genre is about to day; mother of all the WoW clones and the game that defines whether you are a true MMO gamer or not.

- LotRo: by many argued to be the most accomplished MMO as a whole for the time being. Based on an unbeatable IP when it comes to a fantasy MMO, and very well run by Turbine. Some would say a WoW clone. A perfected and independent WoW clone would be an appropriate answer.  

- EvE: The one true Massively Multipliar sand box game out there. Not for everybody. But WoW covers that. EvE contribute to defining the MMO genre by being original in lore and game mechanics. With one large persistent world, where it is all up to you to find your play style, makes this one a must, if you wanna know what MMOs are about today.

- GW: Many would argue it is not an MMO, and therefore GW is on the list. It is the one game which defines the limit between an MMO and other online games. The quality of this game is rarely doubted  and therefore it is perfect to define the MMOs' confine.

Four large AAA MMOs that together make up what an MMO is about today. So I think.

very well thought out.  as much as I hate WoW and what it has done to the genre, I do have to agree that it's the big dog.  You should also put together a top four list of pre-WoW games that made the genre what it is today.

 All mmorpgs are crap these days and might as well just play Dragon Age while waiting for something good.