I pick Valtor because (although he did less damage pyshically.) he did the most damage emotionally. For instance, he gave Chimera the chance to turn Stella into a monster and worst turn her own father againest her, he left poor Layla blind for a while, he let down the tix etc. However i don't really perfere Valtor to the others, I personally perfere the trix.

Valtor is the first!

valtor

valtor only made emotional damage while the trix made the winx and others feel bad emotionly (darcy stole riven from musa) they also destroyed all of red foutian a whole section of alfea, and possibly a bit of magix city.

in the second season icy completly froze pixi village

and in the third season the trix almost killed both flora and her sister.

I picked Valtor because he's beautiful. And a beautiful evil carachter could be more dangerous than an ugly one.
This is my opinion.....

dark bloom and valtor is the most evil.

valtor

I think trix done the biggest damege. Lord Darkar did do eny damege

Valtor is the reason why Domino/Sparx was destroyed

He really knows how to mess with people!

Lord Darkar is obviously the most evil one because he is basically the devil of the Winx Club universe but when it comes at damages nobody can top the Trix.
In season one:
Icy almost killed Bloom
The Trix took Bloom's power away
Imprisoned Griffin
Awakened the army of decay
Declared a war to Magix
Completely destroyed Red Fountain
Destroyed a section of Alfea
Turned Magix into a ghost town(all the citizens of Magix were completely covered in webs by the monsters)
In season two:
Icy killed Sky(but all went good again because it turned out that Bloom has the power to resurrect the dead)
Icy froze the pixie village
In season 3:
Icy broke Galtea's wings
Darcy set the Alfea library to fire.
The Trix almost killed Miele and Flora

^ well put. I'll have to type more tomorrow :P