I've got a favorite from each:
Sydney: The scene where Sydney is driving with John around that beginning. John says, "Can you pull over?" We hear a horn fly by, jump cut to John in the front seat.
Also, in that same scene. John explains why he doesn't use matches. Cut to John waiting in line at the movies, his pants catching on fire. "Whoa! Whoa!" Notice how the people in front of him don't even notice him!
Boogie Nights: Two scenes from here again. The montage that basically describes how successful Dirk becomes. The various characters are reading the article, we see some of the article fly across the screen, we see som clips of the pornos, there's like, four sub screens within the widescreen, and then it all ends with an great dance number
Little Bill shooting his wife and the big stud, and then shooting himself in the face. It was definitely a real pivitol moment in the movie. You know he's gonna shoot her, and the suicide really made me jump. It's even creepier that he smiles right before he does it. Jump cut to a title card; "The 80s"
Magnolia: The first two sections of the prologue. I especially love how the "Greenbury Hill" scene was shot full frame with an old crank camera. That made it all the more better. The Sydney Barringer scene, I love. It's interesting and well done, but I can never get over the badly edited part when Sydney gets shot.
I don't like the Delmer Darrion scene much.
Finally, in PDL, I love when the Brothers crash their car into Barry's. He see's Lena bleeding, walks out, whacks Brother #1, takes his crowbar, and in a matter of seconds kicks all their asses with out much effort. The use of the crowbar... the best Sandler fight ever, followed closely by Bob "The Price is Wrong, Bitch" Barker.