Dr. Strangelove's Lesser Twin
Several posters have commented on how this is one of the best dark comedies ever made. I might agree if it weren't for Dr. Strangelove, which has the same brand of dark humor and irreverent social satire (not suprising, as both screenplays are by Terry Southern), but with much less silliness and fluff.
The Loved One is great, but it's still a lesser twin to Strangelove.