I've just finished Maëlys' story - truth be told, I want to play more with her, but I wanted to see her epilogue, so I played the final quest sooner. After this, I can share with y'all my... it's not really a ranking, more like my opinion on each of my favourite romanceable companions: Minthara, Karlach, and Shadowheart. My Dragonborn is romancing Lae'zel and is still on Act 1, but I don't know, I don't feel the same affinity with Lae'zel as I do with these three. I might do a separate post for her later. (I miiight do one for Astarion too if my second Dark Urge - yeah, yeah, I made a new one - romances him. We'll see if Astarion can survive that far.)
Anyway, here we go:
Minthara

Okay, I said this wouldn't be a rank, but Minthara is TOP 1. Ever since I allied with her on my Dark Urge playthrough, she has quickly become my favourite companion, whether in a romance or not (though both times I had her in my party I romanced her lol). She's a fascinating character who goes beyond the label of "evil" some players attach to her without even giving her a chance. She's ruthless, prejudiced, and power-hungry, yes, but also extremely intelligent, insightful (her opinions on the companions are the absolute best), funny, and surprisingly loveable when you romance her (and if you sided with her on Act 1, the development of your romance has a special flavour).
The Dark Urge romance with her is incredible - I don't know if all companions have romance-exclusive lines with the Durge, but Minthara certainly does, and it's beautiful 🥺 Deep down, she's a romantic poet! And see, that's considering that Minthara's romance line - her character, really - is still incomplete. Imagine when Larian adds everything (and then some) 💜
Also, her sex scene is by far the best in the game, it makes an evil playthrough worth just to watch it!
Karlach

It's fair to say Karlach pushed me to buy the game. I was very excited to romance this muscular one-horned Tiefling with flames all over her. And I don't regret it one bit! Karlach has the best kiss animation out of all the female companions (the others haven't been updated yet, but even when I do, I find it hard to beat the forehead bump 🥺), and her joy at life is so contagious!
Karlach's ending is also my favourite - the one I chose, at least.
Shadowheart

I was positively surprised by Shart's romance line. She becomes so sweet when you start romancing her - if Karlach has the best kiss animation, Shart has the best greetings, and it's not even a competition!
She also has the benefit of having the... I won't say "most impactful" because both Minthara and Karlach have amazing personal journeys in their own way, but Shart's is definitely the most fleshed out.
Honestly, I can't choose between Karlach and Shadowheart. They're special for one reason or another, and I welcome them both in my heart - even though Minthara has conquered it, she accepts to share 😅