Difficult to say withoutknowing your style, but World of Warcraft is the currently most mainstream game out there and it is very casual friendly compared to many other MMOGs, especially the leveling parts of it. I played it for a little over a year, and it was very much fun.
The one I have enjoyed most was proberly Dark Age of Camelot (also my first one), however the game became ruined by to many (stupid) changes, so I left that.
But ask 100 people and you'll likely get 100 different answeres
