What do you mean by best-in-slot gear in that context? highest damage or highest %gold?
gold finder is supposed to be a cheap character with easy to find gear, mine only runs travincal in nightmare difficulty. Your merc does not need forti - infinity (even if that would absolutley work), if you can afford such gear you probably do not need that character.
I prefer act 2 merc with might aura. But the new act 5 frenzy barb with two 1-handed sword is decent as well, suitable swords are harder to find though
Best use a very fast weapon with decent damage, and possibly avoid cold damage to not shatter the corpses.
Bonehew would be my first choice for uniques, tomb reaver is fine but the chance to revive is also a corpse destroyer , same for reaper's toll. Upgraded hone sundan for 45% crushing blow maybe?
For runeword weapons, hustle is stupidly fast, crescent moon with its static field, insight always a decent choice even if you do not need the mana regen. Highest damage would be obediance, but again it has cold damage.
For barb merc you absolutely want at least 1 headstriker unique sword ( preferably etheral upgraded), other one can be another HS, crescent moon , oath, hustle, or even something a liitle more expensive like kingslayer.
For armor, goldskin has decent resists and defense while providing some extra gold, hustle or treachery provide decent atack speed bonus (do not mix treachery with hustle weapon though) , guardian angel can save your mercenary from those nasty hydras. Duress is fine, but cold damage again. Duriel's shell is also a great choice for its cannot be frozen coupled with life and resistances.
For helm, crown of thieves is the most obvious choice with life, leech, fire resistance and gold find. Guillaume's face provides lots of crushing blow for faster kiling . Of course the usual tal mask, bulwark, andy's, vampire gaze wourk for that, Barb merc CAN use barb-only helms , that includes 2 good options : Areat's face, and IK helm.
I essentially am trying to go for highest gold per hour, with some mf to hork, currently running on hell with act 2 might merc, eth reapers with ed/ias jewel, eth upped crown of tieves, eth fort. I spam leap on my barb and the merc just pounds, seems to work pretty good so far.