Spencer Tracy and Katharine Hepburn: These titans appeared in nine films together. You should watch them all, but start at the beginning with Woman of the Year (1942) and see how Tracy is the quintessential American male and how Hepburn is well into her liberated woman stage--which spanned her entire life. Still, her role as Tess Harding is among her best. She, as always, has a timeless appeal. Now skip ahead to Adam's Rib (1949) and see them as married lawyers--the two are at their peak in terms of their onscreen rapport. Finally, jump to the preachy, mawkish, and cheap-looking Guess Who's Coming to Dinner? (1967) Criticisms aside, the duo are wonderful in this, Tracy's final film. I've always viewed these three movies as chronicling the beginning, middle, and end of their onscreen partnership, as if it were the same couple in all three films. That relationship is summed up by Tracy's speech at the end of Dinner. If that speech doesn't bring a tear to your eye, have someone in the room call a coroner, 'cause you're dead, pal.
William Powell and Myrna Loy: Of the couples here, this one is what I'd call "The Well-Oiled Machine." They hadn't the slightest romantic notion towards one another and were often involved in tragic or failed relationships while working together, but these two professionals are probably the best ever, because what they did was act. Show business pros like no other. Watch them in The Thin Man (1934), and note the scene in the kitchen, when Powell is taking a tray of cocktails out and he gives Loy a peck on the cheek as he's leaving. Her reaction looks like the bit was improvised, but the scene is perfect for Nick and Nora's characters, and both actors knew it was part of their magic onscreen. In all, Powell and Loy did fourteen films together, but this series remains their legacy.
Now, go and watch these masters at work. Or if you're already familiar with them, go and watch them again--I know you will.