Oh, this is a very, very difficult question...
I think my favorite pre credit scene would be the one in Scandal. That moment when Moriarty's phone starts ringing... you should have seen my face. Brilliant epic WTF moment... second place (but very close to this) is The Great Game and the grammar nazi thing. And in the third place (very very close too) the beginning of A Study in Pink. I think John saying "nothing happens to me" was the moment I got hooked in this show. The other three are good too, but I can hardly bear by now watching the pre credits to The Reichenbach Fall. 
About the endings... for me the best one is The Reichenbach Fall. Martin Freeman is superb in that scene and the moment we see Sherlock alive... well, I knew he had to be alive, but I didn't expect it to be revealed in that moment. And, as happens with the pre credits scenes, a close second place would be the cliffhanger to The Great Game (but the whole scene... I mean, that "That's what people DO!!!!" from Moriarty froze my blood and made me realise that was a true Magnificent Bastard and not just a cartoony villain) and then A Study in Pink (epic ending to an epic episode).
You see? I can't just pick one 
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Waiting for a crazy man in a blue box to fall from the sky...But the thing is, we've taken away all the things that can possibly have happened, so I suppose the only thing that's left, even though it seems really weird, must be the thing that did happen, in fact. (Miss Marple)
