Hello:
Grandma and Grandpa Longneck is a good couple, the reasons are already written in here. I also like the Tria-Topsy couple, they are no different at all (both have strong personalities, that should be enough

: ) and you really can see that they love each other, specially in LBT12.
Ducky-Petrie: no. They can get along because, well, the entire gang know each other since LBT1, but both of them are the most similar in size, that makes them more easier to draw and to make them interact during scenes, but I do not think they'll have a different sense of friendship like with any other of the group.
Ruby-Chomper: no. They are more like a big sister-small brother relation.
Littlefoot-Ali-Ret: yes. But this will only be solved when they get to the grownup age, but it should be interesting to see.