In my mind, my ideal ending would be either Lee or Clementine dying (or an option for both if they want to do the ME2 thing). It would be dependent on the choices you make earlier and one last choice near the end. It would be a moment of intense drama, either of one of them sacrificing themselves, putting the other out of their misery or a tragic end.
I would prefer the Clementine living option though. We then see some kind of epilogue where Clementine (either older or immediately after the previous event) reflects the choices Lee made. She becomes a hardened survivor, an innocent victim, a morally upright person or whatever it is. Perhaps the epilogue would end with Clem offering a summary on who your Lee was in the game and who he is to her (I would be so moved it Clem had a photo or something of Lee and referred to him as some kind of "dad" if he was good to her).
As much as I hate obligatory "hero needs to die" endings, they work if there is a point! If the emotional payoff, relevance to the theme or final lines nail why it had to happen, I would be fine.