After reading some of the PMQ posts that Pete-zza referenced & a quick Google search, it seems that plain steel (not stainless steel) is preferred by most, with a smaller number of people preferring aluminum. Also pans with dark coatings will usually give deeper browning, although there is a possibility that you could end up with too much browning or scorch your pizza.
Here is a comparison of thermal conductivity of some common metals
055 iron, cast
043 steel, 1% carbon
016 steel, stainless
Although thermal diffusivity is really a better measure of how evenly a material heats
022 iron, cast
014 steel, 1% carbon
004 steel, stainless
Higher numbers indicate better heat transfer. Copper pot & pans are prized by many chefs because of their very even heating & durability. However they are expensive, scratch easily & hard to clean. Stainless steel is infamous for uneven heating, although it is a lot better when aluminum clad. It is durable, attractive & relatively easy to clean. You also have to keep other factors in mind, such cost, ease of cleaning & concerns (by many) of aluminum cooking surfaces. Uncoated aluminum is very reactive. So coated steel or perhaps anondized aluminum may be your best bets.
edited to add thermal info