Graal Online was my first MMO ever and to this day it remains my favorite :<
There was just so much to DO, and desiging your own levels was incredibly easy and a lot of fun. It wasn't even that hard to get them added in a server, I had my own house and everything! Also, the playerbase had a great collective personality about them, and even if you never went on a server's main story quests you'd probably still have a fantastic time. I also liked that every server was completely different; each one was set in the same universe, but they were all unique with their own stories and personalities and everything, pretty much. It was sort of like, the server you played was your homeland in the Graal world. I've yet to see something similar that also doesn't suck.
Another thing I really liked about it was that there was no grinding. It was basically an online Link to the Past, so if you wanted a stronger character, all you had to do was find the appropriate quest. The health gaining system didn't bother with the Pieces of Heart shit, either, you got the full heart container every time. There was also usually one sword and shield upgrade in a server, and the quests for them were always insane gauntlets you'd have to gather 5 or 6 people to get through :<
Considering the limited technology, it's actually pretty amazing what limits they were able to push the game to. My second favorite server abandoned the Zelda-style setting for a modern/futuristic metropolis where constant gang wars raged, and it was incredibly awesome. Bit of a learning curve, but still great! I'd run around as a mad priest with gigantic wax lips, and then go to the church and do hilarious mock confessions. I was a CELEBRITY ;<
It managed to remain fun after the game stopped being free, but not for very long. I went back and tried a trial version a month or two ago, and my favorite server is completely dead and changed from what it was like when I played.
The only other games I've had run ins with are Runescape, Dofus, and Fiesta, all of which are boring and crappy. I still keep Fiesta in the hopes that one of you will fuck around with me on it, but I'm not holding my breath.