This is perhaps the most subjective category of all. When you buy a new car, you can look at hard data to aid you in your buying decision. Things like fuel economy, repair history, resale value, crash tests, owner satisfaction, average repair/maintenance costs, and other factual information can prove invaluable. But when it comes to interconnects, it is not as easy. Rotel does not recommend a particular brand of cable or interconnect for use with its products (there isn't one that's "best"), but we can assist you with some basic pointers. This is really where your local dealer can be very helpful, as they have more experience with using different brands, as well as getting feedback on what their customers have had success with.
Of course, what one person thinks sounds best may not be what you think sounds best, so you may want to see whether or not you can get some cables on a "loaner" basis to try out. It is a very crowded field...there are a plethora of cable/interconnect manufacturers, each with their own philosophy on what makes good cable, each having their own price points, and each sounding different from the other. Cables and interconnects are more important than you might think. If you were to remove all of them from your system, it would be rendered useless. Typically, those that come boxed with the components you buy are quite basic, intended for people who haven't any cables at all. These should be treated the same way you treat that spare "donut" tire in your vehicle's trunk... it does the job temporarily until you can get a new tire on. And when you put that new tire on, you notice the ride, handling, traction, fuel economy, and noise are much improved.
Like new tires, but maybe not quite as dramatic, better interconnects allow you to take full advantage of what your equipment can do. Generally speaking, you should keep your cable/interconnect runs as short as practically possible. Speaker cable should be the same length to each speaker and at least 16 gauge. They should sport durable jackets, be well shielded, and employ high purity copper. Gold-plated connectors will prevent corrosion. When installing your cables, be sure the connections are snug. Keep power cords away from your interconnects, and separate digital and analogue interconnects from each other.