It took me a while. In no particular order:
Ah, the classic, genre-defining rom-com. To quote the Muppet re-make: "I'll have what she's having - but without the pepper."
Between them, When Harry Met Sally and Breakfast At Tiffany's contain pretty much every single trope of the genre: they are endlessly, and needlessly recombined by some kind of scriptwriting machine to create mindless tripe such as Serendipity and You've Got £&*!%#? Mail.
Poor Woody! He tries too hard - and there have been so many mediocre films in the last decade or two that we forget just how much he's capable of. Everyone Says I Love You is Woody at his surprising, understated best, with gentle humour plus moments of perfectly deployed magical realism such as the floating dance scene.
4. Bad Day
The music video for Daniel Powter's song Bad Day is in many ways the perfect rom-com archetype: two characters living parallel lives, destined to be together but separated by circumstance until they start to reach out; great soundtrack; and best of all, it's less than four minutes long. The link above is to the official video. Bonus points for featuring er... Screech from Saved By The Bell.
This film spawned an entire sub-genre of copycat British films - Wimbledon, Notting Hill, and so on - every single one of which missed the point about what made Four Weddings so great. It's not the swearing. It's not the provincial sidekick or the comedy dance routine.
A recent entry and I'm surprised I enjoyed it so much but it's a great script and there's enough cynicism and black humour to give it some bite.
Many films have tried to get this kind of thing right, few have succeeded: Adam is a romantic comedy about a young man with Asperger syndrome. I would be interested to know how it has gone down amongst those with Asperger or other autistic syndromes but for me it was genuine and sensitive although there are a few awkward moments.
Probably the most lightweight entry on this list. However this film plays it's 80s revival card at every opportunity, from the financial backstory right down to the buzz of the doorbells, to great comedy effect.
The definitive rom zom com.
10. High Fidelity
Nick Hornby writes romantic comedies for men. Transplanting this peculiarly English novel about the life and loves of a list-obsessed record store owner to the United States should have been a spectacular failure but John Cusack pulls it off.
Two honourable mentions:
Sliding Doors: a film that attempts to rise above it's rom-com status with a Many-Worlds-Interpretation/Schroedinger's Cat storyline - A for effort.
Brief Encounter: definitely romantic, one of the overall best films of all time, and while it's not a comedy it has some comedy moments.