What, nobody’s mentioned the best ever example of romantic chemistry? Watch any episode of The Addams Family. Morticia and Gomez have absolutely amazing chemistry with each other.
And I don’t think it’s a matter of good acting, per se. There are plenty of cases of an actor who’s good by themself, and who has good chemistry with a variety of co-stars, but who just doesn’t in one particular role with one particular co-star. Sometimes it just works, and sometimes it just doesn’t.
Another great example of chemistry: in the original Avengers from the 1960s-- John Steed (Patrick Macnee) and Emma Peel (Diana Rigg). They called each other by their last names and never so much as touched, but they sizzled.
Negative chemistry. Richard Gere and Jodie Foster in Sommersby.
I find this interesting, because when I was 18 years old, I played Higgins in a production of Pygmalion opposite a 14-year old Eliza. (For those of you who aren’t familiar with the play, imagine My Fair Lady except the two DON’T get together at the end.)
It’s the only thing I ever did onstage where people said we actually showed chemistry. Despite the fact that Eliza and I barely spoke to each other off-stage, and I certainly was not attracted to her, nor she to me. However, we did manage to snap out our lines with adequate fire, look each other in the eye at the right time, and overall seem like we were actually interacting with each other, rather than just reading lines. I guess that’s impressive enough with high school actors that it’s unusual.