I think that Bella and Edward have the most love for each other, because in New Moon Edward heard that Bella was dead, and he immediately went on a suicide mission. To me that shows how much he love Bella, and that proves that he would definitely die for her. Plus, when he left, bella felt like she was going to die, even with Jacob there to "comfort" her, she was still dying inside.
I think they all have a different kind of bond, but all of them are special. If any of the other couples were faced with the challenges that Bella and Edward did, I think they'd all fight for one another. "Wonderful Family!'
See thats hard because you don't see the story from Alice's point of veiw or Esme's or any other charcter besides bella, so I agree with Sheila about the bonds thing: I think it is a tie for all of them!!!
Based on the Twiliht story we must admit it is Bella and Edward; however because we do not know the story of the other characters in detail we cannot compare. What makes Edward and Bella so compelling is that they fall in love in the most unusual cirscumstance and that they transcended all obstacles.
It would be nice if S. Meyer would write for us the love story of each of the couples, including the wolf clan ones.