I've been here a few times, both on my own and with friends, and have really liked New Leaf each time. Lovely ambience, good food, and great Cambodian desserts.
Good range of vegetarian dishes, mix of local and Western food, and good drinks and juices. They have a lot of books that you can buy, and books that you can read while you're in the cafe.
They also support a lot of local charities.