I think for me, it's more about the personalities/dialogue than looks, but I tend to not be a very visual person in general and rather, I hear the scenes in a fic if that makes any sense. It's why I generally can't read hugely long PWPs that are just endless descriptions without dialogue.

I find Benedict endlessly attractive because his voice is just pure audio porn for me. And I find the pairing in the show fun because there's the right level of chemistry, conflict and emotional connection, even though I think the two of them would make a bit of an odd pairing visually when you really think about it.