Well for DA:O, I liked all of the romances. Leliana was my favorite, but I don't think I could pick a least-favorite. They were all enjoyable in their own way.
In DAII, the only one I didn't enjoy very much is Anders, mostly because you could just tell it was going to end badly. And I had a lot of trouble making a Hawke who didn't execute the man, so every time I made a Anders playthrough, it was pretty much going to end the same way: Anders dead at Hawke's feet saying he was glad it was her to kill him. Just not an outcome I want to see over and over. I enjoyed Sebastian's quasi-romance more than Anders'.
In DAI, I've played the Cassandra, Josephine, Blackwall and Cullen romances. The only remaining romance I am likely to play is Solas. But I doubt I will ever get around to playing Iron Bull, Sera or Dorian.
With Dorian my main gripe about him is that he is a m/m only romance, something I just don't enjoy playing. If Dorian were romancable by a female Inquisitor, I'd do it in a heartbeat since I really do enjoy his character and he's one of my party mainstays. But I respect the writers for giving the characters set sexualities and sticking to it. So I can deal.
Iron Bull I enjoy outside of his romance, but every time I try to romance him he just comes off as WAAAAAY too 50 Shades of Grey. The kind of power games and stuff involved in that romance don't come off as sweet or intimate to me, just jarringly creepy. Really not something I'm into. Not going to judge how others see it, though. Just a personal taste thing.
Sera, on the other hand, I just don't enjoy as a character. She's really annoying to me, and I usually find myself either refusing to recruit her or doing so and later kicking her out. Definitely not a character I'll be romancing. Ever. Easily my least romancable character. Even if she were available for male Inquisitors to romance, I doubt I would do it. She's THAT annoying to me.