15 attack shiny that's already the highest level? I'd prioritize that one for sure.
After that I might go for your other two 15 attack gengars, assuming that they are all relatively close in level. Probably the 86% one.
The others I might just sit on as you shouldnt need more than 1 gengar squad but if you have the resources to power them up I would do so in the following order of priority:
Attack > Level > Remaining IV Lvl's