This is a tough topic; so many possibilities come to mind.
There are the classics, of course: the fiery and immature passions of young Romeo and Juliet. The spoiled Scarlett and the smitten, heartbroken Rhett Butler disguised as a rogue. Beauty and the beast. Elizabeth and Mr. Darcy.
And then there are the offbeat stories where love triumphs over adversity. These present opportunities for characters to do something that's quirky, true, but so amazingly special.
Like in "As Good as it Gets" when Jack Nicholson tells Helen Hunt he's going back on his psych medications because, "You make me want to be a better man." Is there a better compliment anywhere?
Or how about "Stranger Than Fiction" when Will Farrell gives the baker, Maggie Gyllenhaal, different packages of flours, instead of flowers? The look on her face is priceless. He truly understood her, and at that moment she knew it.
William Wallace's love and secret marriage. Edward and Mrs. Simpson. Paul Newman and Joanne Woodward.
The first love stories I read were by Kathleen Woodiwiss, the intrepid woman who pioneered the historical romance genre with the 1972 publication of her novel, The Flame and the Flower. I don't know how many times I read that book; but what I loved was the complexity of the plot and the length of her stories - I could revel in the relationships for days.
When I discovered Diana Gabaldon's Jamie and Claire, I was back in that same mindset: rich detail, fierce passion, and a story line not so predictable. And more long books, extending the pleasure, and allowing me to become fully immersed in that world.
To pick a favorite love story is impossible. Even my beloved husband and I have had our rough patches over the last thirty-five years. Raising four children on a single teacher's salary? I don't believe I need to expound on that. Now that they are all grown and on their own, we are having a ball together! But we aren't anything extraordinary.
So what does make a great love story? I believe it's when two people commit to loving each other for the rest of their lives, no matter what. In spite of annoying habits. Or impossible situations. Or temptations that the bed might be sweeter on the other side of the wall.
Honestly, all humans are fallible. None of us is perfect, not a single one. We will disappoint each other, hurt each other, apologize, make up, and forgive each other. But when EACH of us sets aside our selfish (what can you do for me) desires, and puts the other person's (what can I do for you) best interests foremost in our hearts and minds, we can prevail.
And in the end, when a wrinkled, gray-haired, walker-dependent couple can sit together, holding hands, and love the spark of youth they still see in each other's eyes, that is the best love story of all.