From what I found in research is titanium is best, steel next, aluminum is the worse
Depends on your application. Titanium is light and strong but dose not handle the heat from high rates of fire like stainless can. They will start sparking when they get to hot I hear. Even those with full auto ratings. I can't confirm because I don't shoot at a high rate of fire.
Also some use other materials like brass for the blast baffle for longevity.
All of mine are titanium and meet my needs.
NFATALK.ORG has a lot of silencer info and free metering results available.