As this is an opinion question, please feel free to contribute, but please do not remove the answers of others. Thank you.
New Moon, no question about it.
Well, in my opinion its a tie between New Moon and Eclipse.
I think Eclipse is the best.
The best is Twilight because it has a plot. It's the beginning of a romance and love story between a vampire and a human. So sincere.
I know it is Breaking Dawn because so much is happening.
I'm personally thinking Breaking Dawn, although the ending was so anticlimactic that it could've been a second to Twilight. New Moon was just unbearably dull and the never ending moping really made time drag on, and I read it in just over a day. Eclipse was fine, except it would have been much better if we actually saw the battle and some more action, instead of simply getting a dull commentary from Edward who we then barely see fighting Victoria as they are moving so fast they are like blurs.