Ehh Hmm Personally I'd say my favorites Graylu

I would like to see Lisana and Natsu for sure. The reason behind is that we have seen in the beginning how Natsu used to so serious about anything which used to remind about her. He used to blush on her flirting.
I think if Lucy joins in with Natsu it would be cute as well but would be kind of injustice to Lisana's character in Natsu's life.
Juvia and Gray awesome as well..:D
And Jelal and Erza level..:D (y)