Well as a rule of thumb I despise MMO for reasons I've posted several times now on this forum so I won't go into them again, HOWEVER, because I have so many friends who do like MMO I have been introduced to many of them so here are some of my thoughts:
Zero Online - Tried it once, never cared for it. Tutorial was a little confusing, controls were a little clunky, and outside of the mech concept it just felt too much like just another MMO. Positives were that if you like mechs, its pretty much your only MMO choice, and of course its free.
Angel Online - Tried it once, never cared for it. Pretty much the same squabble I had with Zero except I like angels alot more then I like mech's so I really wanted to like this, but at the end of the day it just felt like another MMO. Positives were that it was free and involved angels.
Atlantica Online - Tried it once, never cared for it. I was tempted to give this game another chance but I've just been too busy lately. The fact its not only free, but they'll actually pay you for your level 50 character is pretty ingenious on their part cause in the end the only people getting to level 50 or higher are those that truly enjoy the game, those like myself who found the battle system too rushed and clunky and just the overal game play feeling like "just another MMO" will give up well before 50 and thus never get anything and of course if you actually enjoy it your less likely to get rid of your character for 20 bucks.
Trickster - Actually played this one a decent amount, though I don't have any truly high level characters just a mid level Bunny. When I first played it I didn't care for it but later they improved the game when they started adding Episodes. Though you can still do whatever you feel like and just grind your way to high levels if you want, the episodes give you an actual goal to accomplish beyond gaining levels, help dramatically speed up the level up process, and inject a tiny bit of story and character into what is at its heart still just another MMO. I haven't played the game recently so I don't know how its been improved/changed since I last played but if I had to recommend a MMO this would be near the top of my list.
Rappelz - Played this one a more then most though not nearly as much as Trickster or WoW. That graphics are flat out amazing for a free MMO. This game tries to copy WoW in pretty much every fundamental way so many would say your just better off playing the actual WoW but the highlights of the game are: Unlike WoW its free to start and no monthly fees, unlike WoW every profession can get pets and the game has a very detailed, almost pokemon like pet system for catching, training, and customzing pets, unlike WoW's cartoony graphics character in this game looks very attractive. If your looking for a free WoW or better yet you enjoy WoW and pokemon this one might be worth looking into. In my case the things I hated about WoW were the same things that lost my interest, a lack of real story or direction.
World of Warcraft - Friends forced me to play this one a couple times. Aside from being expensive to buy the game (was 50 back when I played), and insanely expensive monthly fee to keep playing the game (15 bucks a month every month for the rest of your life), now you've got like 70 dollars worth of expansions to get on top of that. As much as I like alteil, if they had that kind of a pricing plan I'd have dropped it in a heart beat. If your a fan of the typical MMO with its endless grinding, no clear goals or purpose, spending hours upon hours waiting for the right party to form or just spent in long boring trips over land/sea you've already traveled across 100 times before AND you've got tons of excess money lying around that you don't know what to do with and would rather give to an already excessively wealthy company then say donate to the needy or some charity then I strongly suggest getting WoW. Oh and did I mention once you get to high levels you get revolve your entire day schedule around the game since you need to keep your schedule free at certain dates and times every week for big guild raids that go on for hours and in the end likely won't even get you anything for your trouble when some one else in the group takes the item that dropped you wanted or the item you wanted doesn't drop at all?
Guild Wars - Not a true MMO but it was pretty good for what it is. You have to pay once to get the game but theres no monthly fee. Theres a strong emphasis on story and quests, you should never have to actually grind. The downsides are that if you don't find the stories that interesting or the unique missions that entertaining the low level cap means theres not much reason to keep playing beyond pvp. PvP is extremely well balanced and one of the games strong points, but for people like myself that just don't care for PvP, it does take away a major part of the game.
Record of Lunia - Again, not a true MMO as you have instanced areas This ones completely free and again this one has a strong emphasis on story and quests over pure grinding. The battle system is set up like an old 1990s arcade game (think Teenage Mutant Ninja Turtles, Streets of Rage, Final Fight, Simpsons, etc) where you move around maps beating the crud out of things and an arrow occasional shows up to tell you to keep moving, but of course you have level ups and lots of new skills to learn also giving it a sort of Fighting game aspect to it as well. I enjoyed this one alot, but when it went from beta to release I lost my character because I mispelled it in the transfer application and now they want me to pay money to fix my mistake and transfer it. I neither want to pay money over something dumb like that, nor start all over again thus I haven't played this in some time now.
Grand Chase - Never quite understood this one, it seems to be set up like a 2d fighter (think Street fighter if ryu moved through worlds), but I couldn't figure out the inputs to do all the crazy special and super moves everyone around me was doing and I've been busy so I never really came back to it. I may come back some day though as it was pretty intriguing and had a format similar to Record of Lunia being free and having an episode by episode level set up.
Maple Story - Played it a bit when it first came out but quickly lost interest do to the lack of direction or quests and the ridiculous amount of grinding. Its kind of a 2d platformer (think mario) which is very unique and the graphics are really cute, if it had some sort of actual story or goal I might have actually liked it, but alas the goal of getting my level higher by killing the same enemies over and over indefinitely wasn't enough to keep me playing.
The bunnies of Lavato have special abilities, like 'Action Skill: Make Carrot Disappear.'