The valve through which the burnt gases are allowed to pass out to the exhaust manifold.
The intake valve will be cooled by the incoming air and fuel mixture. The exhaust valve will run at much higher peak temperatures as it sits in the exhaust gas stream.
A significant amount of cooling occurs when the valve is in contact with the valve seat and the heat is conducted away to the cylinder head. Some exhaust valves are filled with sodium to improve heat transfer.
Intake and exhaust valves must be optimised for mass to operate in high speed engines. The intake valve needs the most attention as it is the largest of the valves. In some engines the intake valves are made from titanium.
The exhaust valve will see extremely high temperature and corrosive exhaust gas.
Piston to Valve Clearance
An intake or exhaust valve will not survive contact with the piston. This makes it imperative to get the timing correct.