I was going to say On the Way to the Airport, too!! That was quite something.
By far, the most amazing chemistry I've seen was in the movie My P.S. Partner (Whatcha Wearin?). The screen was burning half of the time, but be warned- it is a movie with some explicit sex scenes. Apart from those scenes though, even the couple's chemistry over the phone is off the chart.
The next one in line would be It's ok, That's Love. I also thought that Descendants of the Sun, Moonlight Drawn by Clouds and W had uniquely good chemistry. I wasn't even surprised when the lead couple from DotS announced their engagement. In case of the other two shows, there was a lot of speculation that the leads had something going on between them in real life. You can feel it in the scenes.
Also, The King's Woman had an unusual, but good chemistry (it's a bit complicated, but it got there)