Commercial cut pile would be the most durable. Multi color would help hide stains and changes over the years.
Commercial loop pile is durable, but can snag (loop pulls out) and run (long strands pull out) especially if you go cheap. Unitary backed carpets cut down on snagging but cant be stretched over a pad.
If you use a pad you dont want it to be too thick, and you want it to be very dense.
I would NOT use carpet with the pad already on it unless you are talking about high quality commercial carpet. This type of backing is usually called Enhancer. It is very dense and would serve you for many years. A typical attatched pad carpet (kanga) does not have a secondary backing and will often loosen up and cause wrinkling. Not to mention most of them have cheap prime urethane padding that breaks down too quickly.
If you want to make carpet runners, you need to have muslin put on the back of the carpet being used as a runner. The regular backing (action back) on carpet is very rough and can damage the carpet you are trying to protect.
Commercial carpet tile are a great choice, they are put down with pressure sensitive adhesive and can be pulled up and traded with tiles in low traffic areas to even out the wear. But if your not going to actually rotate them I wouldnt spend the extra money.
Depending on the size of the room and the look you are going for you may want to consider some of the glass back, or some call it cusion back, vinyl floors. Some of the higher qualities are pretty thick. They are surprisingly comfortable underfoot, durable, and easy to maintain. Some of the wood and tile patterns are next to impossible to tell they arent the real thing.
Again, thin and firm support is what you want. Too thick and too soft of a padding can actually be counter productive.
Woody
