My first one was Ultima Online and havent found anything yet that comes close to the gameplay UO had.
Ok ok the graphics are outdated now but back then I think it was 1997 it looked pretty awesome.
I ran my own private shard for a year because I wanted to see the other side of a game.
It was fantastic, I was able to create the world I wanted to and even scripted new things.
Going in GM mode and stealthing arround the world and see the players enjoy the world you created is priceless :)