For me, I can't pick between Scar, '18 Shere Khan, Lord Shen and Koba.
Scar appears to lead in the poll so far (having voted myself - multiple votes are allowed, btw). He does have the most brownie points in the 'evil' department, killing his brother and blaming his nephew for it before trying to have him killed too.
'18 Khan has the most 'menacing' appeal to me. His portrayal is the most accurate to Kipling's Jungle Book: physically disabled in one foot, thereby having to hunt man and their cattle. Benedict Cumberbatch (who also voiced Smaug), however, made him no less terrifying.
Shen and Koba are the most bada**, in my opinion. I think Shen's the most unique villain concept, making a peacock just as fierce as a more 'fierce'-regarded animal like an Asiatic lion, a white tiger or a moon bear. Koba's on the same par as far as realistic villainy goes.