I was asked, who was my favorite fictional couple? I've never asked myself this question, making my answer a challenge. I just don't think about books this way, which means that I haven't read those sorts of books very much.
As couples go, I drift away from the perfect pair. Perfect and fated just don't hold my reading interest. The 'finally met his/her match' sort of couples don't interest me because those feel couples feel artificial to me. I like a great tangentialness to couples where you find yourself comparing apples to coffee machines. It's the exploration of the sheer differentnesses, and the unexpected synchronicities, that make a couple interesting to me.
I'll weasle the question by talking some of my favorites. I can't say that I have any absolute favorites.
'Norther Exposure' had a great couple in Joel and Maggie, with each character being irrepressibly themselves, and in that was all the conflict and confluence that you could ever need. Each could easily stand at the center of their own story, and if you put them together, they pretty much created their own plots around any issue.
For sheer twistedness, my favorite couple is Louis and Lestat from "Interview with the Vampire," although this comes in just ahead of almost anything written by Tanith Lee, who always stretched twistedness into some new, inconceivable direction. I'll give a shout out to the Lord and Lady MacBeth as well.
For sheer comedy value, I turn my head towards 'Guys and Dolls,' with Nathan Detroit's rocky relationship to Miss Adelaide standing front and center. They are the epitome of what makes a successful comedic couple. Not only do you want to like them, you see the trainwreck in living color. Lucy and Ricki do a mighty fine comedic job as well in 'I Love Lucy.'
For sweetness, I think of Charlie Chaplain, as he plays his lovestruck fool to his sweet leading lady, Edna Purviance.