The one that we will never have!!!!

Too difficult a question.

What kind of relationship do you mean?
I think for many women the ideal relationship is the one  between a mother and her son/daughter.Or at least that's what they dream of when wanting kids, no.  Isn't it one of the main reasons to have a kid?
For some, the answer is the one between a guy and his mother - that influences how many men choose a wife.
For some the relationship is that of Friendship.
And so on.

So first we need to decide what kind of relationship do we consider the ideal one.
Let's then assume the answer is erotic love between a couple. What is the ideal such relationship. The question reminds me of discussions (and many jokes) of heaven.
What is heaven? Is it a place where you are constantly calm, serious, with no passion? (Only virgins around you?)  Is it a place where fun and passion can exist? There are many jokes of people choosing to go to hell as something actually happens there.
I would say it is similar in relationships. To continue Ram's nice description in this post in the discussion on Woody Allen's Vicky Crisitina Barcelona, which transformed itself into a discussion on relationships, one can choose the calm relaxing relationship. But if one wants passion and heights of emotion, well they can't expect it not to occur elsewhere and lead to arguments (as the movie shows very well).

Sadly, relationship could thus become extremely Hellish, and not necessarily from being overly passionate, but also from a lack of passion and being trapped (like the tree Emily mentions in this post ).
Moreover, if people mentioned how the past appears in our relationships (still in the conversation between Emily and Ram on Vicky), I'm reminded of the book 1984 where people had their own private hell depending on what scares them the most. This is sadly also the potential in a relationship as we tend to conceive of what scares us most (am now thinking of the movie the Sphere), and create our own hell in the world.

The ideal relationship then, passionate in just the right places and calm in others, isn't that exactly the question of Heaven? Perhaps people knowing those discussions can enlighten us on what they came up with? and how to find it, or even create it while still living.

Like Greogry mentions in a post on Do animals have rhythm, where he quotes Messiaen: Eternity is one tone, split it in half and there is rhythm.