I have romanced everyone except Fenris and Sebastian. I am romancing Fenris on my current playthrough.
They each have something appealing to me - For my first run, I Friendmanced Anders because he's just so tragic and is giving up everything for a cause I support.
Isabela is just fun but that's why the final conversation you two share is even more romantic and sweet. You don't expect something like that from her.
But really, if I'm honest with myself...it's all Merrill. It's really ironic in a way because for my first run, I ignored her. Thing is, I don't like face tattoos. They're just plain gaudy to me. But my girlfriend was shocked to hear how I dismissed Merrill as she thought Merrill was my type. So I gave her a shot in my second run. It was very...interesting. I played a hardcore mage-hating Templar-loving jerk who ended up killing Bethany. I'm not any sort of real racist or bigot IRL but I've heard stories about people like that who have these "blindspots." They're terrible and hate-filled 99% of the time but there is that one person they just care about and forget everything else. That's how I saw my second Hawke and Merrill.
I Friendmanced her and I will always only Friendmance her because she is the sweetest, nicest person in the game, if not the whole series. Also extremely hilarious.
Merrill: Your armor is very shiny, Sebastian. Doesn't that make you an easier target?
Sebastian: The Light of the Maker is my armor, Merrill. I am not afraid.
Merrill: Maybe you could ask Him to make His Light less shiny? Then you wouldn't need as much armor.
Or how about when you find Ninette's hand in The First Sacrifice.
Merrill: A person can survive without a hand, this doesn't mean anyone's dead.
God, just reading all this and remembering her voice is making me sigh like some sort of romantic dope.
I can't Rival her. I refuse. I did it in one run and I felt terrible. I was a bully and a monster and Merrill already gets enough crap from Fenris who actively wishes her dead and insults her constantly.
Hawke will protect you Merrill, now and forever.
And I refuse to ever romance Sebastian 'cuz I absolutely hate him.
And as for Tallis, I liked her a lot! Which is why the "romance's end" was a huge disappointment to me.