There is no set Best worst or anything like that...
Mild steel is good.. Handles heat well ND is generally thicker than let's say a 409 stainless firebox
409 stainless allows companies to use thinner firebox material for better heat transfer, it doesn't have the stress corrosion/cracking problems that's present in 304 grade stainless, 409 is like 88% mild steel
304, it does not rust but can still be affected by a cracking type corrosion, go to central boiler.com and you'll see there steel comparison vs 304. I've seen several hardy stoves have to have fireboxes replaced due to this crows feet type cracking all over it
IMO mild steel and 409 are really good......
But please don't start thinking that a thicker firebox is more efficient because it's not.. 409 boxes are usually a little over 1/8 thick. Most mild boxed are 1/4. Test have shown that for each 1/8" you use that you lose roughly 13% in efficiency