What type of metal open vessel/tray would heat up the most if placed in a microwave oven ? Copper, brass, aluminium, stainless, iron, alloys... and any optimal shape/thickness/dimensions that would make it hotter ?

 

-This is not about susceptor films-

Just a guess, but the shape that makes it look like an antenna for 2.45 GHz (or whatever the frequency is) will probably give the best coupling. They all have similar molar heat capacities, so there's no real advantage of heating more per unit energy deposited, though on a mass basis Aluminum wins.

Metals will heat very little (except by sparks...) and resonance will worsen sparks, which appear so readily in a microwave oven. Also, all these ovens have between the cavity and the power source an absorber for the reflected wave, which may prevent efficient resonance of the target.

 

Metals with bigger losses at microwaves:

- Allied titanium, for the higher resistivity... small improvement

- Magnetic material, to worsen the skin effect. Especially nickel layer, iron... Can be very thin.

 

Or cover the metal with a lossy material, say a proper ceramic, or if the temperature allows, a graphite-loaded polymer like silicone or polyimide.