All three were interesting. The "Boogey Man" flashback was interesting, but it bothered me when Blake said, "I love working on American soil, Dan..." even though he didn't know Nite Owl's identity.
How did you come to that conclusion? Dan himself pointed out near the start of the film that the Watchmen (still haven't gotten used to that alternate name) were the only ones to know each other's identities. Maybe there's something in the GN to preclude Dan and Eddie from knowing each other, but I don't know of anything in the movie to this effect.
I liked the Vietnam flashback in the book, but in the movie, Morgan didn't act as if he just had his face slashed while he was philosophizing with Osterman... then... as if in afterthought... "medic!"
No, I didn't like that either. Still, the scene hit every one of the important beats and the scene's three primary actors -- the aforementioned hiccup aside -- sold it quite well.
The Crimebuster's flashback with Veidt is ultra odd since Veidt is suddenly in the Captain Metropolis position... odd enough. But it worked OK, I guess.
Well, Veidt led the meeting in a very different way. The captain had a vision for the Crimebusters and he was very ardent about making it work. As feebly as he attempted to defend his ideas to the Comedian, he at least made an effort.
Veidt, on the other hand, seemed to be a lot more open to the idea that the Watchmen wouldn't work. Moreover, the meeting in the movie seems much more... democratic. Veidt didn't seem to make much of an effort in presiding over the meeting or directing the discussion (rather unlike him, come to think of it).
This, I think, is why the change works. Instead of changing the leader completely, the PTB more or less made nobody the leader. The name change also helped. There's no denying that the movie's Watchmen and the GN's Crimebusters are not the same group.
But the best flashback was Sally's actually. That worked very well. Very well done. Well acted... except for HJ, of course... he was just silly.
It can't be an easy thing to sell an accent or make an impression with only one line of dialogue.
So, back to the question at hand. The rape scene was beautifully ugly, the Vietnam war scenes were wonderfully executed, the riot scene was a touch slo-mo happy but still effective and JDM totally sold the Watchmen meeting.
But if I had to pick one, I would also go with the rape scene, just because it felt like the scene where Snyder added the most and took away the least. Losing the bulk of the pre-rape banter between heroes was unfortunate but inconsequential. The rape scene, as I've said before, was done with unflinching brutality -- perhaps more so than in the graphic novel. Then there's the Comedian's belt buckle, which was a visual touch that would make Gibbons himself proud and envious. And last but not least, we have Hooded Justice and his exchange with the Comedian. That part could easily have been cut or HJ replaced with Nite Owl I, but neither happened.