Ours is in the eat-in breakfast area. We have a pretty big kitchen, and the breakfast area is the back area of it. It's a very convenient place for his pen. Plus, while we're gone all day it is impossible for him to move his pen past the narrow entry between the breakfast area and the main kitchen area, so there are never any messes outside of a controlled area.
I'd thrown around the idea of letting him have the laundry room, but that idea was squashed quickly at the thought of having smells near clean clothes.