Someone change the sign. It's now been "zero days since the last 'ugly lady companion' thread".......
OP: I find Vivienne to be the most beautiful female companion in any DA game. She's absolutely stunning. Cassandra reminds me of Mariska Hargitay, who I find very attractive, even is she's not traditionally 'pretty'. Additionally, although she's not a companion, I find Josephine to be very pretty as well. She's elegant and feminine. Leliana is, of course, always pretty. I don't find Sera to be particularly attractive, but that's fine because I don't think it matters to her how attractive guys find her.