"Body Snatchers," the third screen version of Jack Finney's novel, should have been a contender. In 1956, Don Siegel's "Invasion of the Body Snatchers" emerged as a landmark cheapie, attracting a cult following that still won't quit. Here's the basic plot: Seed pods duplicate humans while they sleep, then liquidate them; the replacements are emotionless, conformist veggies. Some saw it as Siegel's attack on Sen. Joseph McCarthy, the head pod behind the House Un-American Activities Committee, who…