OKEY ..
harry potter - shall always have a special place in my heart as i have loved the books since i was 6 or 7 .. jk rowling is so good, i love all the little double meanings and everything that she puts into her writing that i can only pick out now, like the spells coming from latin n all .. characters are good, love harry, love the plot line, and all the layers that it has .. just an awesomely good and well written series :p
latest obsession, Vampire academy :p - idk .. funnily written, characters are hilarious .. awesome lightish read ..
and yea .. thats actually 10 books, but hey :D

I've just finished Jane Eyre and I'm still reeling. There's so much I loved about it (keep in mind I've only read it once and I read as fast as I could because I was dying to find out how it ended).
I loved the clash of morals and love (Jane had to leave Mr. Rochester when she found out he had a wife) I actually have a favourite quote from another favourite - the Tin Princess by Philip Pullman. "There was love and there was honour and when they clashed it broke your heart."
I especially loved Mr Rochester. I loved how he was a good man who had too much go wrong in his life, even though it was never his fault what happened to him. And because of it he became bitter and unpleasant.
I think I love the Victim of Circumstance theme. It always seems to get me how people become what society treats them like (perfect example the Phantom in Phantom of the Opera). Most of my favourite books have that theme running through them.

It's hard to say which is my favorite book, so I may end up answering this more than once over the years.

Right now, I'll answer for Jonathan Strange & Mr. Norrell: the unique concept and how well-realized it was, to have two very different personalities in a fantasy story which is written as a period novel. I loved how specific and laborious the magic is in the book, and I love the dramatic tension of the different characters played against one another. The craft that went into the writing of the prose and the dialogue is astonishing, superlative.

With my two favourites (Vampire Academy and Mortal Instruments) it's probably the plot and how you will never know what is going to happen next and I also like it how there are fantasy elements and a little bit of romance. I also like a the humour and the hint of action/ fighting scenes. I love the characters and their personalities. I also love the uniqueness of each character.
And... I guess that's mainly it...

Most definitely The Last Olympian by Rick Riordan. It has Greek mythology, which I LOVE (I'm part Greek), it has a romance everyone who has read the series has been DYING for, (YAY PERCABETH!) it has comedy, it has drama, it tells you all the secrets you've been waiting to hear, Nico's mysterious plan, the end of Kronos, and the prophecy about Percy's sixteenth birthday. Plus, it's just plain old well written. Read The Lightning Thief by Rick Riordan, it's the first in the series.