As you can tell from the question, if all three champion ults were placed on a dying champion, which one would take priority. I feel like it would be Zilean's ultimate but Yorick's ultimate is very similar. But if you think about it, would Mordekaiser get his ghost from his ultimate?
Here are some facts about the various abilities:
Yorick's Ultimate Omen of Death: Omen of Death has the lowest priority in terms of revival because it does not resurrect the champion, only revives them temporarily. This means that if an ally were to die while under the effects of Zilean's Chrono Shift or Guardian Angel as well as Omen of Death, the champion would be affected by the one that prevents death.
Zilean's Ultimate Chrono Shift: Chrono Shift will always take precedence over other revival abilities, like Guardian Angel and Rebirth (Anivia's passive).
Mordekaiser's Children of the Grave: If Children of the Grave kills a unit affected by Omen of Death, then Yorick's ultimate will trigger first.
So, Chrono Shift has the highest priority. Next would be Yorick's Omen of Death, followed by Mordekaiser's Children of the grave.
Rift Myths has a video of the interaction with Mordekaiser and Yorick ultimate:
Note that Mordekaiser got a ghost because he killed the Yorick ghoul before his ultimate had timed out.
EDIT: KaelVonrik correctly points out that Mordekaiser's ultimate would be removed at the champion's death, then the champ would be revived by the Zilean's ultimate. So, no Mordekaiser ghost would be spawned. So Mordekaiser would need to use his ultimate after the revive by Zilean, but can cast it once Yorick ults as long as he kills the Yorick ghost before it times out to finally receive his ghost.
WoW, this one is interesting. Let me try.
If Morde ults someone who was ulted by Zilean or Yorick I think that Morde will get his ghost before the target ulted by Zilean/Yorick is revived.
I'm pretty sure Morde will get his ghost and then Yorick will revive you as zombie or whatever that copy is and after Yorick's ult copy fades away champion gets revived with Zilean's ult.
Does it make sense ?
Excellent work by Rapida. Just wanted to add that if the target has additional effects, those would take place as well then that would occur before Yorick's ultimate in the order, as they happen on death, while Yorick ult and Mordekaiser ult happen after death to revive the champ in some ghost form. For example, Karthus passive and Kogmaw passive abilities.