New Oxford American Dictionary...seriously.
thickest book u have ever read (completely)

When I was in Junior High I was a voracious reader and got tired of those little 200 or 300 page books So I went searching for the thickest book I could find in our High School Library. There were two, Gone with the Wind and The Complete Works of Shakespeare. I read GWTW, it was great.
Since then, The Wheel of Time series has 13 books currently, 14th to come out in January. While not really one book, 13 volumes averaging 750 pages is quite a story.

yeah i know who they both were just wondered if it was a good read as that type of book is usually quite dull

It plods, but the many unique anecdotes make it worth it all. One could get by with an abridged version and get it all over with much more quickly.

Reclaiming History by Vincent Bugliosi. 1648 oversized pages with small font (and the index and notes were on an accompanying CD to save space). As an Amazon reviewer said: "It would kill your dog if you accidently dropped it on him/her"

The Count of Monte Cristo is a great book (he also makes a mean sandwich ^^). I've also read the Bible cover to cover.

Well, if multiple volumes counts, I am on page 5,665, book 18 (of 21) volume 5 in the Complete Aubrey & Maturin Novels by Patrick O'Brian.

besides some life science [biology, psychology, chemistry] textbooks, a few of which I have read or almost read cover to cover:
David Copperfield by Dickens was a long read given his predilection towards flowery language, I believe his writing was sold and read by chapter at the time. Anna Karenina by Tolstoy I recall was quite long as well. Not the greatest entertainment value per page I admit.
When I was very young I read several collections of fairy tales and a book that I do not recall much, regarding the flags of the world, which detailed the histories of all the world's countries.
I believe for me it is Herman Hesse's The Glass Bead Game (there is also a title from German which is sometimes used but i don't recall it). It was around 1200 pages. At times it felt more like a chore than a pleasure but all in all a great book (for me at least). I have noticed that with nearly all books there are many different editions with varing number of pages. Although I have some fairly thick chess books i have to (sadly) admit i have never gone cover to cover with any of the thicker ones.
With Hesse's book most are fairly short and a nice read although I would guess that most people who read Hesse do it when they are in there 20s,an adult but a somewhat young adult.(This is when i read about a half dozen of his more popular books...long time ago).