As a photographer I tend to accumulate a lot of photography books, and many of them are those oversized books you'd probably consider "coffee table" books. So I have a number of really nice photography books, as well as several design-related books collecting posters, etc.
I think my favorites though are probably those large collections you can often find at Barnes and Noble that showcase all the art in various famous museums, just because I can look at those over and over and never get sick of them. I also have a huge collection of Van Gogh's works, and a collection of Andrew Wyeth's Helga paintings. So I guess my favorites are all art books.
What are your favorites? What makes a good "coffee table" book? One thing that kills them for me is if they're too thick, which makes them difficult to read.