There are many factors you should consider imo.
As obvious as it is that your ghost will have the most damage from your ad carry (since you have to account for the auto attacks it does BEFORE the champion actually dies and revives)
In many cases its worth more using your ult on someone else.
When the opponents focus another squishy (not your ad carry). Cloning your ad carry is important but I think it's more important to NOT lose your ap carry too soon (before a full round of burst spells for example). So if you see your mage getting focused early don't think about it. Just do it ;)
Sometime Yorick's Ult draws your teammate into commiting and dying without reason. If your ad carry is ezreal for example (or has a flash spell) if you do your ult when he's low you may draw him into a unnecessary death which could end his killing spree. So before you rush your ult on your carry be sure that he has no escape.
You also have to account for the builds of your opponents at the time you are fighting.
For example lets say there's a 3v3 fight going on and you got yourself , your ad carry and your mage and on the other side there is their tank who has lots of armor and their offtank who has lots of armor too and they are low on magic resist. Of course the obvious choice is the mage for your ult.
I could go on forever but the main thing to remember is that there is no better ghost. It depends on what kind of damage you need and when you need it.
Personally the most Devastating Yorick Ult I've done was not with an Ad Carry. It was with a fiddlesticks who got caught in a teamfight. He managed to hold on for a while with his drain and I ulted him. When he died we kept fighting 4v5 but he revived in the bush next to the fight (where he died) and ulted right away melting their whole team..
An ad carry could not possibly have done that much damage to their whole team that fast.
PS. I basically agree with Toast I just expanded it a bit.