I don't necessarily equate favorite romance with favorite romanceable
character, so this might be a bit convoluted. Iron Bull might be my favorite character in the franchise and he's a LI, but I don't usually romance him. Not because he's not a smooshy marshmallow who brings the feels (HE IS), but because Sera's romance arc struck a particular chord with me and I'm just as happy being his friend as his lover. So anywho, favorite romances:
DAO: Zevran. Not my canon romance, but by far my fave. I love when the assassin falls in love with the mark - the romance feels extra satisfying knowing that it started off so very rocky. I love his background and the vulnerability it brings to his character, how he's willing to get his hands dirty. His personality gelled really well with my fierce, loyal Tabris. The earring offer felt especially poignant because she was still carrying around her fiancé's ring, and she felt it was time to move on. I dunno, really... this romance brought all the feels. Asking the Warden if they want him to stay and what his place is, and basically giving him a sense of self-worth he never got with the Crows... it's lovely. And when he finally admits that he'd storm the Black City itself for you, I
believed it.
DA2: Fenris, with Isabela as a close second. I love Fenris's romance for a lot of the same reasons I love Zevran's - the vulnerability and hidden strength, struggling with what it means to be your own person in a world that wants to own you. He's fiercely loyal and unapologetically devoted to Hawke, and freely admits that Hawke's the most important thing to ever happen to him. Once he wrestles with his feelings your relationship is solid. That boy ain't never gonna leave your side again. Feeling worthy of that level of love and devotion was pretty great. And now I can't kill Hawke in the Fade because,
"Promise me you won't die. I can't bear the thought of living without you."DAI: SERA SERA SERA. Hers is quite possibly my favorite romance in the series. The greatest gift you can give her - and this is the sweetest thing - is being
proud of her and your relationship with her. She's been looked down on all her life for being poor and uneducated, an elf but too human, never good enough until you come along and treat her like something special to be treasured. She's hugely affectionate with her nicknames and schmoopy journal entries, her silly but heartfelt gifts. If someone hurts you, whether it's Corypheus or some racist Orlesian or whatever else, they're getting an arrow in the face. She's a secret (or not so secret?) romantic. She's just... the greatest, ahhh.
edit: I think my running theme is devotion.

LI's who would break a jaw for you, hahaha.