If My Heart had Wings is a great VN to start off with. If I would recommend any visual novel for the first VN to read, it would be that one. Second, Katawa Shoujo. The story and the pacing is really good and the developers deserve praise for their work.
When I first read through Kono Oozora, I thought Kotori would be my favortie too, but seeing as she is just the tip of the iceberg, I decided to stray and try for the others. All of the characters have great personalites and mostly all of the characters (Ageha... enough said) have great routes that blend in lovely with the story and setting.
Follow up Kotori's route with Amane's route. You won't regret it. She's my favorite now... and not just because of her big oppais... well mostly, but not all of it. Oh who am I kidding... look at my fricken picture icon...