You can make a character powerful without him/her being instantly sexy, if their true power is inspirational. Vader is popular, The Emperor less so.
I have a villain in my fantasy series, who at first seem to be your run-of-the-mill genocidal cult leader.
But when you get to hear his side of the story, the perspective changes.
He is genuinely trying to create a world governed by brotherly love - where the children of orcs and the children of elves are playing in the meadow. Where the swords are broken and the shields shattered. A world where every specie lives among each other in peace.
But to realize this utopia there can be no place for the warmonger. There can be no time for strife.
Everyone in the world is invited with open arms to join the new world order, but those who cling to their old, warlike ways must go.
So before the world can know peace, there must be a last, great war.
Whenever his armies encounter a new tribe, they are invited to join up. If they refuse, they are slaughtered. Total genocide.
And it is not a sham. Within his growing dominion life is better. Tortur and rape are outlawed. Women are considered equal to men and the peasant has the same basic rights as the nobleman. He routinely executes his own people, when they break those rules and commit warcrimes and yet he lives by them.
When his followers prepare a feast for him, he refuses and have it sent to the orphans of war. He has sworn off the attention of women and lives in celebracy because he fears the corrupting influence of power and is afraid he would some day violate a women too afraid to say no.
He is not sexy or actionminded, but he is terrifying because you GET it. The reader understands his vision and would totally be behind it, if it did not involve global genocide. He is the face of fascism - Ghandi and Hitler combined - but because he keeps himself humble he doesn't become larger than life.
Sorry about the rant - I just really like the guy.
Hopefully it was somewhat helpful.
WormHeart