Some of my all time favorite Classic Romantic Comedies :(There are so many good ones, it will be hard to narrow it down. I am putting down some of the less known, worth watching gems.)
Mr. and Mrs. Smith : Hitchcock's (no surprise, one of my all time favorite directors) only comedy starring Carole Lombard and Robert Montgomery. Their chemistry was spectacular. Even though this wasn't considered a blockbuster and I doubt anyone has ever seen it, it is truly a gem.
Bell, Book and Candle: Starring Jimmy Stewart and Kim Novak, two of my favorite actors, this films about witches, spells, and the magic of love. It is an odd little film with a lot of charisma.
Adam's Rib: It is always fun watching a movie on the battle of the sexes. Starring Katherine Hepburn and Spencer Tracy, this duo's chemistry is unmatched by any other couple. It is just a fabulous film that is ahead of its time.

Other great Romantic Comedies (the well known list): It Happened One Night, Some Like It Hot, Singing In The Rain, Pillow Talk, Bringing Up Baby, Roman Holiday , etc.

Other Genres of Classic Films: I don't usually seek out watching Thrillers, Sci-Fi or Film Noir, but there are many films that I love from these genres. I will break them down according to directors.

Hitchcock: I simply love the films that he makes. I have seen all but a handful of the films that he made. My favorite is Vertigo but all of his films are wonderful. Here are highlights of some of my favorites:
Rope : Eight 10 minute reels, this film looked like it was one long continuous take. The story is excellent and the symbolism superb. Based on a true story, this movie was about two "friends" who killed a third for the thrill of it.
The Birds : You would be surprised at just how scary birds are when they attack.
Rear Window : Looking out and peeping at your neighbors have never gotten more interesting as this.
Shadow of A Doubt : How well do you really know your relatives?
The Lady Vanishes : First you see her, then you don't. Are you crazy? Or is it everyone else?
Secret Agent : Maybe this was where James Bond got all his ideas - espionage and falling in love.

Kubrick : His films tend to move quite slowly but the two that I like are simply spectacular.
2001: A Space Odyssey : Technology taking over the world. Hmmm.. is Kubrick a prophet or what? Technological Dystopia, here we come!
A Clockwork Orange : Humans without decency, soul or sympathy. Yet again, how did Kubrick know? (The book by Anthony Burgess is better - redeeming)