Fatal Attraction Starring Michael Douglas and Glen Close. If I remember correctly this was the first movie made about this besotted mistress theme. Play Misty For Me followed or was it before? Anyway FT rates highly in my book as does Play Misty, both 9/10
Play Misty For Me came first by around 15 years or so; it was made in 1971 or '72. And it wasn't really "besotted mistress," but "besotted wannabe mistress." But as you say, pretty good.
Blazing Saddles
9/10
I imagine that most of you are familiar with this one by now, but if not, be sure to check it out. One of the greatest of parody films ever, although it loses a little comic snap towards the end. But before then you'll be in stitches at the antics of Cleavon Little as Sheriff Bart, Gene Wilder as the Waco Kid, etc.