Just one? Oh crap.  My choice would have to be Some Like it Hot with Tony Curtis, Jack Lemmon and Marilyn Monroe.  The premise, the dialog, and the details are all superb. After accidentally witnessing the Valentine's Day Massacre in 1929 Chicago, Joe (Tony Curtis) and Gerry (Jack Lemmon), a saxophone and a bass player respectively , catch a train for Florida with an all girl band, posing as Josephine and Daphne.  On the first night on the train, Gerry/Daphne covers for Sugar Cane the band singer and movie jiggle-interest, played by Marilyn Monroe when she's on the verge of being fired by Sweet Sue, the band leader.  Later that night, she climbs into Gerry's pullman berth to thank him/her:

Sugar:  I just wanted to thank you for covering for me earlier, tonight. You're a real pal! If it hadn't been for you, I'd have been kicked off this train in the middle of nowhere, sitting on my ukelele!

Gerry: Oh it's freezing out there! They agree to have a nightcap on account of the cold, and Sugar fetches her flask, then returns:
Sugar: Climbing back into Gerry's bunk:  Here, this'll put hair on your chest!

Gerry: Whoops! No fair guessing!

 Tony Curtis  as Joe/Josephine is a hoot when mimicking Cary Grant  on the beach in Florida as he pretends to be a millionaire complete with yacht to lure Sugar, who in turn pretends to be a runaway society heiress as they commiserate about the high society life:

At the start of the conversation: 

Sugar: which one is your yacht, the big one?

Joe: Indignantly turning on Cary Grant:  Certainly not! With all the  unrest in the world, I believe it's immoral to own a yacht that sleeps more than twelve!

(Still an amazingly timely movie!)

Joe: It just gets tedious, doesn't it, the balls the cotillions --

Sugar: Riding to hound, debut balls...And always with the same four hundred!

In the meantime, Gerry/Daphne has won the attention of an elderly and presumably short sighted millionaire dirty old man, Osgood, played by Joe E. Brown  who actually owns the yacht Joe/Josephine is using to impress Sugar.

On the night of the big date, Gerry/Daphne convinces the millionaire not  to spend the evening on the yacht, and end up spending the evening doing the Tango together at a road house, leaving the yacht free for Joe and Sugar.  The audience sees Joe and Sugar both hastily getting ready for their date after the band's performance at the hotel; he desperately taking off his dress and feminine disguise and dress as the millionaire guy Sugar will be expecting.  Then they're both hurrying toward Osgood's speedboat, she on the upper dock level, Joe frantically pedaling a bicycle on the lower level, reaching the speed boat just before she does. As Joe waves, he realizes all of a sudden, that he's still wearing his long sparkly earrings and yanks them off just before Sugar reaches him. The scene alternates between Joe and Sugar and Gerry and Osgood,  with rapid camera cut aways.  Late at night, or early in the morning, Joe  returns to their shared room to find Gerry still dressed for his night out, lying down on the bed waving his marracas. (Part of the humor of this scene is  lost  now that same-sex marriage is no longer considered preposterous, but both Lemmon and Curtis are still funny in their timing and line delivery.)

Gerry:  Joe! I'm engaged!

Joe: Congratulations, who's the lucky girl?
Gerry:  I  am! Waves marracas some more.

Joe: Are you crazy? This will never work!  What are you going to do on your honeymoon?

Gerry: Oh, we've discussed all that.  Osgood wants to go to the Riviera, but i think we should go to Niagra Falls instead.

Joe raises predictable objections to what will happen when Osgood finds out he's married another man.

Gerry: Look I'm not crazy, Joe, I don't expect this to last.  But after a quick divorce and a generous financial settlement, I'll be set for life!

After a second escape from gangsters at the end, Joe, Gerry and Sugar are all fleeing in Osgood's speedboat toward his yacht and Gerry tries to call off his engagement.

Gerry: I have to level with you Osgood, we can't get married!

Osgood:  Calmly steering speedboat Why not?

Gerry: Uh, I have a terrible character. I've lived with a male saxophone player for two years!

Osgood: I forgive you.

Gerry: I smoke! I smoke all the time!

Osgood: I don't mind.

Gerry: Oh damn it, Osgood! (Tears off his wig) I'm a  man!

Osgood:  shrugs, unperturbed  Nobody's perfect!

Gerry stares into the camera in utter desperation.

                                            The End

If I could have a runner up it would be George Stevens' A Place in the Sun With Montgomery Clift and Elizabeth Taylor.  I'm a sucker for romantic tragedies, and looking at beautiful people on screen. And those two are quite a celebration for the eyes, separately or together.  Mongomery Clift had a definite "Boy from the wrong side of the tracks" appeal, and Elizabeth Taylor at age nineteen is damned easy on the eyes.