It's amazing to see how all-over-the-board these answers are!
For me, I'd be equally happy with either Viv/Solas or Viv/Blackwall.
I'd be okay with Blackwall/Solas, because I want to romance them both, but I would feel guilty, and would prefer Viv to be a LI.
I like Varric a lot, but of the 4, he's the one I least want to romance, so Vivienne/Varric would be my least favorite scenario.
Then again, i rather suspect that Dorian will be my new Fenris and thus almost impossible not to romance on every playthrough, hahaha. My gf already makes fun of me hardcore for how much I love Fenris.