OMG there are SO many good ones. These are the ones I will watch any time they're on, and ask my husband, I can recite the dialogue along with the actors
Hopscotch (Walter Matthau, Glenda Jackson)
Father Goose (Cary Grant playing against type)
The Goodbye Girl (Richard Dreyfuss and Marsha Mason)
Barefoot in the Park (Jane Fonda, Robert Redford)
Becket & The Lion in Winter (Peter O'Toole as Henry II in two fabulously written screenplays)
To Kill a Mockingbird (quintessential Peck)
The Thin Man (William Powell and that "lanky brunette with a wicked jaw" Myrna Loy)
Singing in the Rain
Airplane! (what can I say, I'm a fool for puns)
Also, guilty pleasure, the original Airport with Helen Hayes
Meet Me in St. Louis (love the period details)
The Women (and just about any other movie from 1939 except Wuthering Heights - see
http://members.aol.com/grbmd/year1939.htm for a listing)
I'm sure there are many more but I can't pick just one and can't remember them all.