i think icy is the most powerful because she seems to have the most moves and they are very strong.
also if winx wasnt a childrens show there are quite a few ways for icy to kill someone if she wanted.
I think Icy because she is the one that everyone is most afraid of, so she's powerful in the social world, and she has powerful magic. BUT i think Stormy might have the most powerful attacks if her powers were put to full use BECAUSE she can do tornadoes and stuff like that.
icy might be scary to others and has nice ice attacks but shes not the most powerful and neither is stormy thanx to her not thinking be4 she acts she messes things up alot and she does have wicked storms but darcy can hyponotize almost anything to do her evil bidding while she can make ppl blind and hit them with dark energy magic shes not just an illusionist shes awsome
I think darcy is the most powerful she is the witch or queen of darkness you may say as she an hypnotise or make magical shields . though Icy is the eldest her attacks and speed comes slowy .so darcy is my fav and also my answer