Too many ways to go with my final pick, so I'll just take the first movie by my favorite filmmakers.
Ebert put it best:
"The genius of "Blood Simple" is that everything that happens seems necessary. The movie's a blood-soaked nightmare in which greed and lust trap the characters in escalating horror. The plot twists in upon itself. Characters are found in situations of diabolical complexity. And yet it doesn't feel like the film is just piling it on. Step by inexorable step, logically, one damned thing leads to another...Every individual detail seems to make sense, and every individual choice seems logical, but the choices and details form a bewildering labyrinth." They build crazy walls with sensible bricks."
It's a great modern film noir from 1984 in which Dan Hedeya hired M Emmett Walsh to kill his wife and her lover. After saying to Dan "how can I trust you not to go simple and do something stupid" and hearing back "the money," he agrees to the job. But then he decides to kill Hedeya instead because that's just one murder with 0 witnesses rather than 3 (he'd have to kill him anyway to eliminate the witness). You'll have to watch to see what happens next, if somehow you haven't seen it already.
So I'm taking
Blood Simple as my final movie. If you like the Coens or noirs and haven't seen this, then do yourself a favor and watch it.