As Yorick, when should I ult someone other than the ADC?
Since Revenant scales off AD, and also the most valuable member on team who can help a lot more with extra 10 seconds of life-after-death is the ADC - it seems a natural fit to use R on Vayne or Ezreal etc. when the team is around.
Are there any cases when I should try to ult someone else on the team, if so how do I choose the right target?
In addition to AD carries, it can be useful to use Yorick's ultimate on Tanks and Bruisers.
Tanks generally have some CC and are difficult to kill, so bringing them back to life for 10 seconds allows them to continue to protect the squishier carries.
Bruisers generally have good health and damage, so bringing them back gives extra damage to team fights and a ghost that can't be immediately killed.
It can also be useful to use Yorick's ultimate on allies that are about to die but haven't used their skills. If your allied Morganna is about to get nuked, allowing her to come back to life gives her a chance to still use her ultimate. Though this doesn't take advantage of the ghost, it does prevent a fight from immediately becoming a 4v5.