My friends over at MPOGL.com put up an article evaluating very MMO communities. The five games covered are Guild Wars, Maple Story (Scania Server), Halo 2, FlyFF, and Survival Project. Here's an excerpt:

By looking at the pro's and con's break down you can already see why I rated Guild Wars number one for this article. In all my years of online gaming I've seen many different online communities, and this one has amazed me the most. I have seen many games in which their community is filled with polite people, but these games are usually small and fan made. Considering the size and publicity of Guild Wars it is amazing that there are only trace amounts of jackasses. Notice that I never mentioned this people every game will have jackasses, it's a fact, but in Guild Wars it seems it has less than a moderate amount of these people. Just enter a town or outpost and people are generally having fun or starting a party in the middle for all to join. Ever witness people spamming hate in the chat lobby in games like Survival Project, etc...? This rarely happens in Guild Wars, which is good because spamming/flaming really takes a lot from a community.