I'm gonna list all the obvious ones, but only because they're some of my favourites too.
The DaVinci Code by Dan Brown is a very well written book, incorporating a tonne of religious information with a great plot. I found it a tad bit slow going in but after that I could barely put it down. If you can get past some of the religious talk and terms, it's a really great book.
Angels and Demons, also written by Dan Brown, also follows along the same sort of lines as The DaVinci Code, although it isn't so overbearing with the religion and has a bit more of an intriguing plot, imo. Some of the concepts mentioned in the book are quite interesting and I stayed up until 3 in the morning reading this book simply because I continuously wanted to find out what happened next.
Finally, and I'm sure it's been mentioned several times - I saw it listed on the first page already - is The Dark Tower series by Stephen King. The man is simply a sensational author and this series is by far the most compelling, plot thick, mentally stimulating series I have read. I'm half way through Songs of Sussanah at the moment, but I can say without a shadow of a doubt that it's filled with stunning visual imagery and such attention to detail that I can just about feel myself moving through each chapter of the story as I read it. The tale is also pushed forward by amazing characters that make the story feel even more real. I started with The Gunslinger at the end of school last year, and when I went on holiday for christmas last year, I went through 4 more, looking for the next installment in bookstores like the series was a drug.
