Last Seconds Shock

Both "24" and "Lost" are shows which are enjoying huge fanbases that I am sure are overlapping to a large percentage. While the format and setting is very different, both shows use the technique of the "last seconds shocker" to great effect and there is a reason it works well for both.
"24" with its continuous action and fast pace can actually turn a single trivial moment into one of suspense just by the sound of the seconds ticking away. The conspiracies and intrigues that surround terrorism plots can certainly provide a variety of shocking moments that are perfect for closing episodes.
"Lost"'s elaborate background and tons of characters, on the other hand, has a vast wealth of details that can be revealed. The writers have proved very skilled in their ability to prepare and present these moments just at the right time.

The question is who does it better?
I don't want to venture a general answer, so I'll just comment on the latest episodes. On "Lost", we get the first glimpse of a new major character's background, Libby. It is shocking for two reasons. First, it is the first direct connection between two characters after the countless indirect connections between them. Second, Libby is proved to have lied on at least one occasion which raises questions. The shocker's value is undiminished since the viewers don't have much information.
On "24" the things aren't nearly so good because the shock is too big and the viewers do have some information that could make them call shenanigans on the writers. The previously helpless, indecisive and weak President Logan appears to have worn a mask and now the writers have work to do to convince us of their plot consistency.
No matter what, both shows have lots of people waiting anxiously for the next episodes. If you are not one of them, you should think about joining them!