What is the best cable to use for DSL?

Cat6 cable is the very best. There is a Cat7 in the works but it is not yet readily available and the connector is different. So if you get Cat6 you have the very best available that will work on most everything. Coax DSL cable is superior but mainly used in industrial applications. A Cat6 shielded cable is also quite effective. In a pinch i have run two Cat6 side by side and punched the blue and orange of the first and the green and brown of the second cable on a single jack. This prevents crosstalk thus data loss. The Cat7 cable is a joke. It does not use an rj45 and is heavily shielded as to almost make it impractical.