MMOs tend to have dedicated fan bases and everyone has their favourite: Aion, City of Heroes, WoW, Guild Wars, Lord of the Rings Online ... the list goes on and on. Everyone has their own personal favourite or the hip one that everyone and their pet goldfish is currently engrossed in. For example, I'm renowned for my adoration of the gorgeous USA MMO that is LOTRO, as well as my passion for WoW while LOTRO power leveling.
Personally, these are the only two MMOs which do it for me enough that I will spend copious amounts of time exploring, questing and flying around pretty aimlessly. I enjoy beautiful scene and epic storyline while Lord of The Rings Online Power Leveling. So two is my limit, I will occasionally play a couple extra, if I have to review something or a beta comes up but there's often little to keep me going past that. Yet LOTRO and WoW remain, filling the void and I couldn't be happier. Especially as there's more than enough happening between the two of them at the moment to keep me very busy. However I know that lots players love fresh feelings. They may change the games very often, for the new questing or free beta.