That’s easy: any contest that has a book as a prize!
Paperbacks are great but I’m happy with ebooks (because then I don’t need to figure out how to fit it on my bookshelf).
My other favorite contest is any contest that’s open to international readers. A lot of the contests I see advertised have interesting prizes like cute quilts or monogrammed mugs, but they are only open to US entrants. Not that I blame the authors – the cost of postage could be prohibitive, and there’s every chance a mug would never arrive intact.
