I like them all except for Facebook.
I just recently figured out how to use my Twitter (xD) so I really enjoy it so far. It's a great way for me to keep in touch and ramble about all the little stuff. :]
I've had a Myspace for years now, and I love my page, but I'm not signed in constantly. I check it every few days.
I have multiple Youtubes. xD I love taking videos, but it's a real pain to upload them. And I don't really watch videos on Youtube, I just listen to music.
So right now, probably Twitter, but overall, probably Myspace?