Morrigan is easily my favourite, I explained this somewhere else before so I'll just copy paste it.
Honestly I think Morrigan's romance was one of the best in games. In a lot of games *cough* Mass Effect *cough* the romance is just building up to sex and that is it rather than looking at how the relationship between the characters impacts them both and importantly their relationship doesn't exist in a vacuum and rather is impacted by events of the story at large. With Morrigan's romance sex isn't the endgame because well, sex isn't the endgame in a romantic relationship. You don't just have sex and then that is it. The relationship has problems, real problems, not fake bad rom-com end of the 2nd act misunderstanding where she thinks he did something bad so they break up rather than talk about it for 5 seconds but then she finds out he didn't do anything and they get back together, now they are happy forever, the end. You can have sex with Morrigan early enough in the relationship, rather the romance actually deals with the characters' relationship with each other on a emotional level. Morrigan undergoes a character arc as part of the romance/friendship she has with the Warden (I look forward to seeing in Inquistion how Morrigan is different depending on how the Warden interacted with her in Origins, they could just cop out and have her be the same regardless but I hope not) as she really opens up more and it becomes clear that she isn't just a heartless ***** who is incapable of liking anyone. The relationship ends up being tested with the Dark Ritual and no matter what it ends pretty tragically in the base game. Witch Hunt does more to conclude it which I really did like, but regardless of your decision at the end of it, they were still apart for 2 years. I like the dialogue in Witch Hunt where the Dalish you are with mentions you fiddling with the ring Morrigan gave you, personally I put the ring on the moment I got it and didn't take it off.
Then Alistair is second for me.