Sorry, couldn't resist. The right answer is, it depends. Wider the better on loose gravel, roots/rocks and washboard, where you can let out a bunch of air. Hardpack or relatively smooth roads or trails allow you to go skinnier. In my view, it's all about volume. If I had to choose a good compromise in 700c, 35-40 would be nice for both on and off road, but ideal for neither.