I voted for the White Witch. She is very powerful in many different ways across several worlds for many centuries.
She is very clever, able to use words to manipulate Diggory in MN and Edmund in LWW.
In Charn, she had the magical knowledge to end all life on a world with just one word. In Narnia, she mastered the knowledge necessary to perform magic in that world, becoming strong enough to work an enchantment on Narnia so that it was winter for one hundred years. She was able to turn her enemies into stone in Narnia, and into dust in Charn.
She was able to use a sword and dagger with at least some degree of proficiency in LWW, and in England, she was able to balance on a cab as if it were some sort of chariot. In Charn, she was able to wage a war against her sister that lasted long enough for the population of that world to be seriously depleted and the capital city to be overrun with blood. In Narnia, she was able to conquer Narnia for her one hundred year winter reign, which probably involved some military exploits. That makes me think that she definitely has experience fighting on the battlefield (can defend herself and can attack her foes quite effectively) and also has some degree of skill orchestrating long scale military campaigns. That seems to be supported in LWW when she organizes the army of the dark creatures and leads them into battle against those loyal to Aslan.
In my opinion, the Witch is clever, magically knowledgeable, and able to wage a bloody battle no matter what world she is in, so, for that reason, she is the villain I would least want to fight, because she is strong in so many different kinds of fighting in so many places over such long eras.