XLR & Spades: Which is best? Pure copper, plated gold, plated rhodium or ?

Silver plated Copper is the best choice. Silver plating oxidizing is of no consequence as Silver Oxide is almost as conductive as Silver is itself. There is NO reason to ever use Brass. Or gold plating. Yes its shiny and pretty but silver is a much better conductor and Silver Oxide  is better than untarnished gold. Choose solid Copper connectors with silver plating. Or solid Silver connectors with Solid silver wire. Always use Solid core unless movement is unavoidable. If you want to know why buy a solid core copper speaker cable and then buy a stranded cable of like size and length. Bet you $20 even your wife will be able to tell the difference listening to them. This is seldom talked about but try it before giving an opinion. I think you will agree.

 Jeff

My preference is for silver plated copper - it provides better transmisson/clarity and faster dynamics

The tarnish does not bother me because

1. it only appears on the visible parts of the connector that is not forming the actual connection
2. reseating will re-establish a perfect connection
3. I do not clean my connectors because it makes no difference to the operaton or sound
4. I do not use any type of jointng compound 

I am happy with my sound and see no advantage in obsessing over tarnish, because most of the connctors seldon tarnish in the atmosphere of my home

I have tried other types of metals and platings and prefer dynamics and clarity of the silver plated Copper

Regards - Steve

Gold is the standard due to it's resistance to oxidation.

For me base material matters the most, then coating mix.

I always find that pure copper as base material and silver/rhodium plating as coating works great.

What is best, depends on your system, whether you want to push things, mellow or refine.