The box should be placed for maximum privacy, away from noisy appliances, and with an easy "escape route," in case you have a cat who likes to bully other cats. Being trapped in the litter box will surely lead to litter box avoidance by the victimized cat. If you are cramped for space, a screened-off corner of a room will work well. If you want to spend the extra money, there are some very attractive pieces of furniture that also conceal litter boxes.
However, unless your cat has a preference for covered boxes, or the box is in an area where you'd like to keep it inconspicuous, such as the kitchen, the best box to start with is a plain rectangular box. Make sure the box is large enough that your cat can move around in it comfortably, with no "overhang." This one, by Van Ness is the "Large" size, 18.375" wide x 14.875" deep x 5" high.