For me the books are best when the tension and obstacles are character driven. That's why I like the first half of Breaking Dawn better than the second half--because in the first half, the characters are driving the story by making tough decisions and finding themselves at odds with each other (or not, as Jake and Edward actually find themselves on the same side). That's great story telling. The second half of the book is plot driven and doesn't hold my interest as much (though I love the Volturi, I though the story could have stood on its own without the buildup and showdown.)
For that reason, I think that New Moon is my favorite because it is character driven tension and very little of the threat is external, it's all about the characters and their inner struggles, not their external threats.
But I like all the books, and I think all of them have a lot of that character driven element that I love. Because honestly it's the characters that sucked me into the saga in the first place.
for me it is because the wedding they finally get married their honeymoon and then they have a baby that is where things get tense I love it that bella and jacob are just friends I love it that edward and jacob finally get on good term's they all get there happily ever after and it could not have ended any better than it had plus I love the getting ready to fight with the volturi all that is very tense related it is just simply awesome book
I think its a tie between Eclipse and Twilight for me. In Eclipse I just love the "love triangle" between Bella, Edward and Jacob. I also like how we see new born vampires and we get the stories of Rosalie and Jasper. Plus Victoria is finally killed.
In Twilight, I love how Bella and Edward fall in love and I like how the relationship between them builds and grows. :]