John Wayne and Maureen O’Hara in The Quiet Man certainly acted like they had actual hots for each other.
William Hurt and Kathleen Turner in Body Heat, for amazing chemistry.
Jennifer Aniston and anyone, such as Jake Gyllenhaal in The Good Girl, Jim Carrey in Bruce Almighty, Clive Owen in Derailed and especially recent ex-b/f Vince Vaughn in The Break-Up. I guess she had chemistry with David Schwimmer on Friends, according to fans (although I never was one), but on the big screen, she’s failed to spark any excitement in any of her pairings for me.
–Beck
Also I hope I am not the only one who thought the chemistry between Robert Forster and Pam Grier in Jackie Brown was smouldering…damn, I was hot for Max Cherry in that movie!
–Beck
The movie that had the most on-screen sizzle for me was Natural Born Killers. Juliette Lewis and Woody Harrelson, neither of whom I normally dig all that much, were HOT together. I totally bought into the premise of this movie because of their chemistry. Oliver Stone always walks the line for me between compelling and pretentious, but his two leads definitely made this one great.
Sizzle:
Lana Turner and John Garfield in The Postman Always Rings Twice.
The Snooze:
Nicole Kidman and Tom Cruise in Days of Thunder and Far and Away. Its a wonder they were together as long as they were.
I’ve never seen Veronica Mars, but I can’t imaging those two could be even half as sizzling as the best TV couple ever. Of course I’m talking about Dick Van Dyke and Mary Tyler Moore as Rob & Laura Petrie. 
You have a point – but the Bruce Willis/Cybill Shepherd chemistry in Moonlighting was also prettty intense.
Rob and Laura were pretty good, but the best TV couple ever was James Gardner and Mariette Hartley in a series of commercials for Polaroid cameras.
That Taster’s Choice coffee couple radiated chemistry, as well.
Fleishman and O’Connell in Northern Exposure.
and, same series, I forget their names, the gay couple.
Ditto. And I always thought Shelly and Holling had some convincing chemistry too, especially considering their (vast) age difference. Their romance should have seemed icky, yet somehow they made it work.
The Country Crock couple leaves me cold.
Waitaminnit - wasn’t that Giles from Buffy the Vampire Slayer - Anthony Head?
What matters to me is how legit the communication appears to be between the characters - could be cute, hot, angry-on-its-way-to-hot - as long as they appear to be really communicating. I remember when Hepburn says to Tracy “Hello you!” in - what? Woman of the Year? - it just seemed so familiar in the best sort of way.
And as for the May-December romance thing, I agree - but there was an extremely cheesy episode of ST:TNG featuing Jean-Luc getting reluctantly hot n’ heavy with a then-unknown Famke Janssen that actually worked.
How about that hovering around the telephone scene in “It’s a Wonderful Life”?
I’ll third Before Sunrise and Before Sunset.
I always thought that Chris Cooper and his lady were perfect in Lone Star. Their sex scene – about 30 years of pent-up desire – was excellent.
I know I am lame for mentioning this…but Sarah Jessica Parker and Chris Noth had incredible chemistry on Sex and the City. So much so that whenever she switched boyfriends you could tell she AND the audience were getting bored. Their sizzle made the storyline that (well, an admittedly unlikeable protagonist) she’d go back to such a cad over and over again pretty realistic…in a weird, co-dependent unhealthy sort of way.
I’ll also add the actors who play Jim and Pam on The Office (American) as having great chemistry…except they’re not the leads since it comes off more like an ensemble show.
In terms of WORST chemistry I’ll 2nd the Old Man Bond and Catherine ZJ pairing in that movie (was it Entrapment?) but it still doesn’t compare to the looks of loathing that really hot french babe was giving David Spade in that movie where she’s a violinist and there’s a dognapping and he loses his best friend’s wife’s diamond. I think her name is Sophie something or other…I’m pretty certain she was also a Bond girl. You could really tell she hated him.
Fred Astaire, in Daddy Long Legs and Funny Face. Kermit the Frog would have been more convincing. (In the latter, at least, he seemed to have some chemistry with that annoying woman who wasn’t his love interest. Actually, that just makes it worse, doesn’t it?)
As for GOOD chemistry…Kurt Russel and Goldie Hawn in Overboard is cheating, right? 