Danzo for the following reasons:
1) Strongest Wind user so far, not to mention he could use Izangi and had Shishui's eye. Simply said he is one of the strongest ninja in the manga, personally I believe the only reason he lost was because plot demanded him to, also lets not forget that he was holding back against Sasuke because he needed to reserve energy to fight Tobi afterwards, in the end it seems he held back too much and payed the price for underestimating Sasuke.
2) Completely selfless, everything he does is for the betterment of the village. He even commits suicide without hessitation in order to prevent Tobirama from getting Shishuis eye. Its not easy to have that kind of conviction to do whatever it takes to protect the village.
3) His relationship/envy towards Hiruzen, "You were the leaf, showered in sunlight. While I was the root, obscured in darkness." These are his dying words and they speak volumes to the kind of man Danzo was. Always striving and fighting to prove himself to be better than Hiruzen. Not to mention that he named his Special ANBU force "Root", which means that this feeling of resentment towards Hiruzen has been something eating him up for quite sometime, possibly even before Tobirama made Hiruzen Hokage when they were kids.
He is one of the few characters that is earnestly and wholeheartedly complicated. He is a bad guy for all the right reasons. He has given up his concious, emotion and personality for the sake of protecting the village and in return he is given no thanks, and he doesnt care, he didnt do it for gratitude, he did it because it is the right thing to do. Most other characters feel one dimensional.
