It's easier to Friend Carver than Rival Bethany. I believe I read it's literally impossible to Rival Bethany, at least to max it.

Anyway, Varric is the hardest one to Rival for me. Need a guide for that trout.

I just 20 Rivalry from Anders just from the end of Enemies Among Us. It is refreshing to make such quick progress. Him and Merrill and Fenris are the easiest to figure out.

In my own current run I'm romancing Fenris for the first time. In order, I've done:
Anders
Merrill
Isabela

They're all great for different reasons although Merrill is still hands down my favorite. I'm interested to see what becomes of Friendmancing Fenris.