If we're talking about "kissing" movies, The Princess Bride is the greatest romance film ever made.
I like how critic Mark Cousins attributes "romantic" to films that seek to fill your senses, and evoke a sense of awe, in the same way that Romantic painting had done in the early 19th Century. It makes a ton of sense, and helped me to come to the realization that, for all the talk of photography, it's actually the essential step across the threshold to motion pictures that allowed for a true transition away from and a replacement for monumental painting. It really is no accident that painting began to move quickly toward abstraction in 20th Century Modernism, coinciding with the rise in popularity of the movies. Critics have been talking about this at least as far back as Walter Benjamin, and it's super interesting.
At any rate, in relation to the question posed by Guy, a strong argument could be made that just about anything produced by Hollywood in the last 100 years could be considered a romantic film. Heck, even without the academic terminology twisting, the Hollywood formula, and its influence on narrative formulas throughout the world, generally includes elements of romance, and in that way, even the most bombastic action flicks could be counted as romance. That's why the other greatest romance movie ever made is The Fifth Element.
I'm also pretty partial to Love, Actually even though I have to skip past one of the most horrible and stupid "nice guy" scenes ever shot -- Rick Grimes Holding Signs.