1st valan , no need to say why!
2st driller , old hero but she is still good in arena with skin+ awaken , also very useful in sky forests. in some hard stages 20+ floor she's your saviour!
3st nightshade , not that op in arena like before but still good with skin , again also good in sky forest in some stages
5st embraeal- salus(if you have him 6 or 7 star )
and the heroes i think that you shoudnt get their skin at all :
vernus ( vernus is op for hol and raids , but his skin ability not do anyting for bosses , so its a waste
franzicoopter ( get a skin that only reduce 100 points phy def?? no thanks! )
soren and alecia are useless i think too but not sure
thats all i khow i wish it helps
from server 63!
and sr for bad English