I'll try to be serious about this...
The best is likely Twitter, mostly because I've used it since it started...
The worst depends really, but in no particular order...
Google+ (a wasteland of nothing, I'm waiting for Google to make it vanish into thin air)
YouTube (Nothing but leet and whining)
Facebook (No, I really don't know, or care, that you think I might know someone... and I don't)
LinkedIn (Professionally it's almost barely useful, otherwise it's a waste)
Pinterest (Why is it that someone HAS to join to see my page?)
I could rant about others... But I really shouldn't....
Tera