Ok well my fav would be all of them (lol) but if i had to pick one...i would say the first, Twilight. Here are some of my reasons:
Though i love all three books, i love Twilight best because of the fact that it is the first book. It was not the second or third that got you hooked. Really, think about it, the first book is where we first meet Edward, Bella, the cullens, the little twwn of forks, Charlie etc - who you manage to fall in love with despite yourself, you are first shown their relationships, how they talk to each other, or act around each other. Twilight was really well written and the characters were developed really well, I love reading about the forming of a relationship which is what Twilight is, the forming of relationships between people which in my opinion a first book should do - from the relationships between people and introduce conflict.I have other reasons as well - but i got fake nails for my formal and its weird/hard to type so...yeah lol =)

anyway - yeah - thats my opinion, i would LOVE to hear others...

thanks
xxx