Gerry Adams of Sinn Fein must be a candidate. He took his party from being a discredited mouthpiece for a terrorist organisation to being one of the leading parties in Northern Ireland. A man who, whether you agree with him or not, is entirely dedicated to the cause of his party and has managed to lead it largely intact through its various changes. He also commands the grudging respect of his political opponents, though you'd scarcely hear most of them admit it.
There's something about Alex Salmond I really don't like, though that may just be me.
All party leaders seem to be defined by their image, which is rather unfortunate. It's hard not to despise Nick Clagg, however. I like Nigel Farage as a person: unfortunately his views are mainly ridiculous :-)