Muffy x Rock or Muffy x Griffin.
I'm actually not the biggest fan of Muffy, but she makes such cute couples with these two I think. Rock and Muffy because...well an easy guy with an easy girl just makes sense to me? And Muffy and Griffin, because Griffin is so sweet and treats her like she should be treated. And I think she deserves someone good after all her terrible relationships.
But other than that I pretty much like Jack with every girl.
Jack x Ann
Jack x Karen
are the best though.
Oh, and Doctor x Claire is cute, too.
But possibly the BEST ever is Toby x Candace. They would be just way too adorable together in my oppinion.