Which is the best material to create a magnetic field?
Silver has the highest conductivity. For AC applications skin effect can come into play. Copper wires coated with silver are quite often used and litz wires (insulated bundles of very small diameter wires) are a further improvement and can counter some of the associated problems of skin effect. 

To do what you are trying to do effectively usually means operating frequencies between 100kHz and 1MHz and highly resonant parallel tuned circuits. The inductance (the coil) needs a very high Q factor and the reduction of AC resistance is usually key to this.

With high Q factor coils in parallel with a resonant capacitor and a suitable driver circulating currents in the coil can easily be tens of amps and this might be the level of field strength you need to make this work at a distance of maybe 5 cm. If you want bigger distances and significant powers (tens of watts or above) this is tricky for a home spun project.