A stainless steel chimney cap is an excellent choice, though it is not the only good one. The choice of which chimney cap to install depends on the type of chimney, the desired cost and aesthetic concerns.
Every active chimney should have some sort of chimney cap to ensure that sparks from the fireplace or stove do not ignite a fire on the roof, as well as to keep birds, squirrels and other animals from taking up residence in the chimney, potentially introducing flammable materials that can start a serious fire. In addition to stainless steel, chimney caps are commonly available in aluminum, copper and galvanized steel. Aluminum and galvanized steel caps tend to be the cheapest but also less durable than stainless steel.
Chimneys intended to handle the exhaust gases from burned fuel oil should definitely be covered with stainless steel caps, as they are much more corrosion-resistant than galvanized steel and aluminum. Additionally, stainless steel can be painted black to effectively hide soot build-up. Copper is the most expensive and requires some additional care, but it is often considered the most aesthetically pleasing and can last many years with the proper maintenance. Copper can be kept polished or allowed to naturally develop a green patina.