The Cessna 337 Skymaster is listed here because its a real IFR aircraft that is very suitable for business travel across the mountains. The twin turbocharged engines give this aircraft a high operating cost, but its performance and cost is certainly competive with other twins. If you look at the 337's performance in high altitude regions, it offers some impressive figures. The Turbo 337, for example, has a single engine ceiling of over 16,000 feet! Many non-turbo twins won't maintain half that altitude on one engine. Some of them won't even be able to maintain a positive rate of climb at the field elevation of many of the Front Range Airports. I know which one I'd rather be flying when I'm IFR over the Rockies at night when an engine goes out.