A Severe Mercy by Sheldon Vanauken. A true story that reads like a novel. It's about about soulmates (married) who travel around the world in a yacht, come to Christ through their friendship with C.S. Lewis...need I say more? I read it like 5 times in college when I was on the quest for "the one." It does have a tragic element but the spiritual one counterbalances it (hence the title)...think Shadowlands but more romance, beauty, poetry, etc.
The Enchanted April by Elizabeth Von Arnim. I saw the film first and fell in love with it (it's finally on DVD in the USA as of this spring! We had a petition circulating) Eventually I read the book and enjoyed the extra details. The book and the film are equally good, in the same way that To Kill a Mockingbird is (but of course a totally different genre). The romance (s) is central, though subtle, and it's hard to beat the setting of four women in the 1920s on holiday in a castle on the Italian riviera.
My favorite mythical romance is of Beren and Luthien in Tolkien's Silmarillion, but the book falls more into the fantasy epic allegory genre than romance. It has everything and that's probably one reason I liked it so much.
I'm not really into romances (though I did adore Pride and Prejudice, as well as the BBC production; disliked the Keira Knightley one), so a book/movie has to have other elements for me to totally be drawn in. I also rarely reread or rewatch anything, and I've done that with both of the above many times (for Enchanted April, just the film, but I know I'll reread the book also).