Man, X-men has so many great and complex villains (or antagonists...as often with XMen they are not really villains...I don't view Magneto as a villain, for example).

As far as pure villains, I'd say the Demon Bear from New Mutants was pretty awesome.

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Also for villains I liked all of Iliana's rivals from limbo. All of them were so great for so many different reasons. Sy'm is literally the exact thing an 8 year would be completely terrified of so he was the perfect monster to harass her when she was a child.

And on top of that, his power apparently included being able to keep the butt of a burning cigar in his mouth...no matter what happened to him, it just stayed in his mouth...he could get punched through a wall and it would look like it flew out of his mouth but in the next panel it would be right back between his teeth. This photo below is right after decapitation:

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And Belasco, another foe of Iliana (X-Men name Magik) was even cooler. Look at this panel with Kitty Pride (you know the artist was laughing his butt off when he drew this up):

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Emma Frost is certainly a key anti-hero or antagonist (even at her worst with the Hellfire Club i wouldn't view her as a villain necissarily).

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Yea, her only problem was just dressed way too conservatively.

Yep, just think how high his over head is to run that giant hidden assination chamber. Just those androids alone would be so cost prohibitive.