Best:
Excuse me, you dropped this...(works pretty well provided they actually have dropped something and you actually picked it up)
Don't I know you from somewhere?
Worst:
Are you [insert name here]? Ok, I am sad to say that your family have all died of uber-cancer...could I buy you a drink to help you get over it?
(for someone famous) Woah! You looked better on TV...forget I just said that.
So (pauses while pointing at woman's breasts) how much do you bet that if I dropped a coin in there I could fish it out?
Er...you do know that you're bleeding, right?