Myspace of course...
Speaking of which:
http://www.myspace.com/yummy_muffins
^_^
The level of customisation is much higher, helping to make your profile much more personal. And also resulting in you liking your profile more, since you can make it look more or less how you want, as long as you know how.
Facebook has the whole "application" thing going on, which is good. But doesn't stop all profiles looking identical.
Bebo is just an all round amateur version of the two.
Which is evident by the fact it's nowhere near as successful.