Material selection of heating element

Consider that ohmic heaters are usually made of thin wire, and that the majority of the current flow going through any wire is close to the surface of the wire. Both these things mean that even if the wire itself might be a poor conductor of heat, that heat has only a very short distance to travel before it reaches the wire surface and gets carried away by conduction, convection and radiation. Because of this we wouldn't expect the wire's thermal conductivity to make much difference.

Even if it did, consider the following case where we assume the hot wire is quite nonconductive thermally. This means that the heat generated in the wire can't get out easily. This means the wire gets hotter and hotter with time, until it begins to glow with heat. At that point, the primary heat transfer mode becomes radiative and the heat escapes anyway.

Not really much to choose from : platinum , nichrome ( with modest variations like 0.5 % Al) and for some high temperatures ,silicon carbide . Platinum tends to disappear on weekends when a facility is shut down . So the practical choice is nichrome whatever the conductivity. The decision is wire diameter and coil configuration.

1. For maximum power efficiency, you want to match the load impedance (i.e., the heating element resistance) to the source impedance (i.e., the resistance of everything else in the circuit, including the power supply). This is the maximum power transfer theorem. (Volumetric heat generation is $J^2rho$, where $J$ is the current density and $rho$ is the resistivity. With a lower load resistance, you're not taking advantage of the circuit current. With a higher load resistance you're dragging down the circuit current.) Choose your material (and its electrical resistivity) and its geometry (cross-sectional area $A$ and length $L$) accordingly, considering that the linear resistance is $R=rho L/A$.

2. The presence of thermal conduction means that the temperature profile of the heating element will be parabola- or catenary-shaped to some extent. To suppress this profile and obtain a fairly uniform temperature, choose a material with a high thermal conductivity, maximize the cross-sectional area, and/or minimize the heating element length. In addition, choose connectors with poor thermal conductivity to avoid bleeding away heat.