That depends on where you live and what your priorities are. If you live in a high snow area, then a very durable roof with a lifetime warranty might be the best. If you live in a high snow area with also a high fire potential during the summer months, then a standing seam roof, which also likely has a lifetime warrantee as well, would be your best bet. The smooth nature of the material allows the highly flammable dried debris from neighboring trees to slide right off, dramatically reducing the chances that during a fire, this material will catch fire and lead to you entire home burning down. These are very real concerns and where we work in California, forest and wildland fires are a real concern and these types of roofs are mandatory in any home surrounded by wildland.
If you life in a really hot area, where the passive solar heat gain from a dark roof would be a real problem, then getting a roof with a high reflectivity or albedo (how well the roof reflects the suns heating rays- 0.00 to 1.00, the higher the better) and high emissivity(how well the roofing, once heated, emits the heat rather than retains it, in hot areas, the higher the number, ranging from 0.00 – 1.00, the better) are essential. Many roofing manufacturers state that their roofing is a cool roof, but comparing their emissivity and reflectivity numbers is really the best way to determine the true characteristics of the roof. Energy Star (U.S.-EPA and DOE) ( http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=RO) produces certification of cool roofs that takes into consideration the steepness of the roof, and the buildup of dirt on a roof after three years and both the reflectivity and emissivity of the material itself. In California, the Cool Roof Rating Council (CRRC) administers the certification of cool roofs that are sold or installed in California in adherence to California’s Title 24 energy efficiency standards requirements.
There are several companies that make all different kinds of cool roof derivatives of their common roofing materials. Some cost a little bit more, but many of the asphalt based cool roof derivatives can cost 30-35% more than the non cool roof versions of the same roofing material. While this is great to have the options, my experience is that people don’t want to pay the significant premium nor do they tend to like the lighter colors of these roofs. There are also cool roof versions of concrete tiles, light weight concrete and metal roofs.
One of the best cool roofing for commercial buildings and residential building with flat roofs are actually sheets of fiber reinforced white PVC membranes that can be chemically and heat welded together to form a impermeable tub of water that is so strong, it is capable of withstanding ponding water without leaking. The white color reduces the absorbance of heat so much that it has a 0.90 reflectivity rating. The most common manufacturer of this material is Dura-Last. While your question was specific to sloped roof, I bring this material up because it is a much better option for flat and low sloped roofs than the typical hot mop or roll roofing that is typical of these roofs. These materials typically have a 20-25 year warrantee.
We do not recommend wood shakes for a multitude of reasons. They are a natural, carbon based material that undergoes the common degradation problems that all natural materials like these do, in that they degrade in sunlight, allow mold and fungus growth, are a substantial growth medium for moss growth, come from forests that are cut down to produce the materials, are flammable and fragile and do not wear well with foot traffic.
The concrete, light weight concrete and tile roofs are also lifetime roofs, barring damage from the errant tree limb, misplaced foot, etc. These materials can also have cool roof characteristics and are much easier to walk on than wood shingles and standing seam. They are more pleasing to the eye and do not degrade with solar and weather exposure. The main problem with all of the materials, is that they are significantly heavier than the other types of roofing materials and often, especially in earthquake country, the house either has to be designed to withstand that load on the roof, or the whole roof structure must be structurally retrofitted to hold the load. This can be a potentially significant cost.
As far as the insulation value, the roofing material itself is too thin to be considered insulation, no matter what the material. Insulation is measured in an R value, which stands for it’s resistance to heat transfer, or insulative value, per inch of material. The highest R value of the best insulation, a closed cell polyurethane foam, is R-7/inch. You want more insulation, put more inches. Roofing is too thin and too poor of an insulation. For example, the typical roofing material is 0.05”(standing seam metal roofing, R-0.02) to 1.0” thick(concrete tile – R value 0.08/inch), resulting in really no insulation value, where the typical new home construction have walls insulated to R-19.
Hope this helps.