Well, I already have a Giratina, and I already have a Griseous Orb (the question is whether your Giratina is untouched, the one I received is up three levels from the catch point and I know I can someday put in the untouched one).
As for the others, I have the Shiny Lv.5 Larvitar Event. Yanmega...may interest me; I have to see if my Yanma knows AncientPower, and if so, it'll be there in one level. If not, it saves me the trouble of using Heart Scales. I think my Yanma was caught JUST as it forgot AncientPower in the moves learned list.
So depending on the stats of it, the Giratina is possible (with or without the Orb, I guess with wouldn't hurt), and the Yanmega is a backup option.