The most impressive residence I've seen around here is Edward Doheny's Greystone Mansion (and even then, I only saw it from the street below). In fact, it's so vast that I'm having trouble finding contemporary photographs that properly impart the scale of this place:
LAPL (not dated)
LAPL
It's in Beverly Hills, so it's not technically L.A., but it's still a hell of a building. The Tudor architecture belies the fact that it was completed (relatively) recently, in 1928.
Of course, Greystone had its own "noir" part to play in local history: In 1929, 36-year-old Edward "Ned" Doheny, Jr., married father of five, was the victim of a murder-suicide committed by his personal secretary (and suspected lover) Theodore Plunkett. Below is an illustration of the event overlaid on a crime scene photo:
LAPL
Below: Greystone Mansion today
(Wikipedia)