This is not a black and white answer as it really depends on personal preference. Obviously the most populated MMO as of late has been World of Warcraft, but many would disagree that this truly is the best. Here is an excellent to-the-point article that will answer your question. You can purchase here as well: http://bestmmogame.com
Check it out and see what you think.
A really good MMO in my opinion is Forsaken World or Warhammer.
As of 2013, there are fifteen of the good adventure FTP MMO suggested online. Some of the names of the good adventure FTP MMO that are suggested are Eden Eternal, Grand Fantasia, and Age of Conan: Unchained.
aq.com is a good one
i did is so good
nothing
There are many places one might play MMO (or better known as MMOG) games. The "Free MMORPG List" website or "MMO Bomb" websites would be a good place to begin.
try Pokemon indigo at: http://www.pokemonindigo.com/
depends how u like it :p there is alot of brower mmo's one of the best i think is Runescape but try out Aqworld Battleon Dragonfable
1. RuneScape 2. Lord of the Rings Online
Dead frontier,definitely. DayZ Zombie Pandemic Die2nite avoid WarZ ( a lot of problems around that game) check link for more details and a bigger list of zombie mmo games http://www.zombieplace.com/zombie-mmo
http://www.mmobaseball.com/createaccount.aspx?ref=6314 is a good baseball mmo. If you join from the link above, you will get 300 free points.
Currently the biggest MMO is World of Warcraft