All of them at some point.
I'm doing Cullen first because that's always been the goal, plus I'd rather do it first just in case his romance arc does feel lacking in comparison to the companions since he can't come out in the field with us etc. I'm so used to DA2's romances now that anything would be an added bonus in comparison, so it wouldn't ruin my first playthrough of DAI if I do it whilst still unaware of what the companion romances are like... if that makes sense?
Oh no, I get what you mean completely. While I have some worries in the back of my head I have truly began to put full faith into Bioware, it may be reckless but oh well life isn't fun if you just sit around and be stagnant. I think the biggest thing with Cullen's romance will be the factor that he is not going to be with you out in the field so that kind of...battlefield lovers feeling is a little lessened, but at the same time I think the Devs have really fleshed him out and made his romance as satisfactory as possible.
I was at first going to romance Vivienne but since we can't I think I may run two PTs side by side, one Inquix going for Cassandra and another for Cullen.