Stranger on the Third Floor is a 1940 film noir starring Peter Lorre and John McGuire. Elisha Cook Jr. is also in this, and it's always good to see him. Whether or not it's actually noir is debatable, but that's what Wikipedia says.
Noir of the Week calls it "The Film that Gave "Birth" to the Style called Film Noir". Time Out calls it "a remarkable film". DVD Talk calls it "a masterpiece in miniature." Rotten Tomatoes has a critics consensus score of 86%.