I think which resource is best may depend somewhat on whether your capital is a castle or city...

Gold deposits seem to be particularly sensitive to the level of development your capital city is at...

I just tried doing a test on my russian campaign, turning the FoW off and mousing over all the various trade resources... Timbuktu remains the best location available, but while switching my capital from Moscow (City) to Bulgar (Castle) increased the value of the amber deposits my merchants are actually on (Due to distance increase) it decreased how much I would be able to get from the gold in Timbuktu.

It would appear the formula is somewhat complex.