For me it would be Perfect World!

It was my first ever mmo around 6 years ago now and I fell in love with it! Especially the music.
I recently (well a few months back) tried to go back to it to go back to some of my favorite places and just feel generally nostalgic but so much had changed! I tried to replicate my old character as much as I could (as thankfully the character creation remained pretty much the same) but all the starting zones had changed and that kinda made me go "oh.." and quickly lose interest! But maybe I will go back to it yet again another time!