I know this is a somewhat overly-specific category but I think most of us here have seen at least one film where we felt the first half was vastly superior to the second half? I'll give the two examples which immediately came to my mind:

1. Lawrence of Arabia - I think pretty much all of the iconic scenes and lines of dialogue come from the first half which ends brilliantly with TE Lawrence basking in glory when he returns after his successful raid on Aqaba. The second half seemed to me to have run a bit out of steam and the Ottoman torture scene seemed to drag the movie down to a crawl.

2. Full Metal Jacket - I don't think anyone can deny that the first half on Parris Island is what has become iconic and etched in pop culture. The second half in Vietnam still has some very good scenes but it just doesn't carry the same emotional impact as the first half in my opinion.