Hardcover offset-print book, with a pdf backup copy.
Hardcovers look nice and can take some punishment. Offset-print books are put together well, and the bindings feel sturdy. Having a pdf backup allows me to read the book conveniently. I can take a tablet to someone's house when I run the game instead of heavy (but pretty) book.
Softcovers are nice as well. I like how flexible and light softcovers are. PDF and ebooks are great in that they are portable. I'll be honest, any book is welcome in my home.
I do like the effect that print-on-demand has had on the RPG hobby. Now we can get physical copies of games that are out-of-print or that have niche audiences. Print-on-demand is of a lower quality that offset printing, although I have been mostly pleased with the titles that I have bought. One downside of print-on-demand is that it takes the books out of stores. We may lose some people who might have picked up that strange looking book in the store and decided to give that weird game a chance. However, our society is changing and POD may be the way of the future for printing.
