What's the best plating material for interconnects plugs ?

Clamped OCC copper!
From your list, probably silver. I believe oxidized silver is as good a conductor. I’ve seen copper connectors. Not sure how they hold up with all the air. Gold is probably okay for longevity but it’s not a great conductor. Sounds warm. Rhodium is great for detail but can sound a little hard and chrome-like.

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.