What's the best plating material for interconnects plugs ?

Just to clarify where several posters have stated that silver oxide is a good conductor, That is simply not true. Silver oxide (Ag2O) is not a conductor but a p-type semiconductor with a band gap of about 1.2 eV, so conductivity is about 1/1000 that of pure silver.

However silver oxide does not readily form on the surface of silver. The typical tarnish on silver is silver sulfide (Ag2S) which is also a semiconductor with a band gap of about 0.9 to 1.1 eV. So for both the oxide and the sulfide, the conductivity is poor compared to pure silver.

choice is clear, silver it is ! thank you all for fantastic write ups and all of you help !

Most small RF connectors are beryllium copper, tellurium copper, or brass with a gold plating for corrosion resistance. Same for audio connectors. While gold is not the ideal conductor, it's better in the long run than other materials that decay and corrode. 

Just to clarify where several posters have stated that silver oxide is a good conductor

@ljgerens Thank you for the correction. Ironically, I read that misinformation on Audiogon.

In my experience it's down to how good the contact is and whether it gets disturbed. If it's clamped tightly and rarely moved then copper or sliver are fine, if it's a press fit (like RCA, XLR or banana) then gold on gold eliminates worries about corrosion and the softness of the material ensures a good contact surface area. There is no benefit from using rhodium outside of a caustic environment.