So far, I have:
Josephine/Qunari: Josie is the most lady-like LI we've had, and is all about fluffy and cheesy romance. Combining that with a race that's stereotypically seen and brutish and gruff is very entertaining. Double points because my Adaar is a mage too.
Sera/Elf: This was my very first PT, so I didn't know about Sera's feelings on elves at the time. It ended up having a lot of friction and drama, but I actually liked that in the end. It gave the romance and PT a very unique flavour, which I doubt will be repeated in the other Sera romances.
Cassandra/Human: I really wanted to romance her with a Circle mage, so human it is! I love the idea of a high-ranking Chantry member shacking up with a mage, and seeing their discussions on things like the Circle and Chantry.
Iron Bull/Dwarf: Interesting height difference, that's all!
Solas too. Lined up but not played include Dorian/Elf, Cass/Elf, Josie/Elf, Blackwall/Dwarf, Cullen/Human, Sera/Qunari. I didn't vote on the class poll since it's single-choice only.
Since people are explaining their reasonings, I prefer Trevelyan/Dorian because Dorian's had an awful lot of heart ache regarding sex and romance and I love the idea of him finding his Prince Charming rather than a smuggler, mercenary, or nomad.
Oh, I hadn't considered this before. I like the sound of it, might give Dorian/Trevelyan a whirl!