This is a very subjective subject. Americans might say "Abraham Lincoln" or "George Washington" or "Thomas Jefferson". Britons might say "Winston Churchill" or "Gladstone" or "Disraeli". Germans might say "Otto Von Bismark" or "Frederick the Great". Mexicans would say "Benito Juarez", while some other Latin Americans might name Simon Bolivar. Then you have the statesmen of ancient times...Charlemagne, Augustus Caesar, Solomon, the Yellow Emperor, etc...
So let's not fight about it. ;-) Just pick who you would choose, and explain why. It could be interesting.
One thing that is clear. Politicians who were commanding the winning side in major wars usually get praised to the skies and turned into the nation's greatest heroes. Those who were on the losing side are usually damned for all time. Is it necessarily because they were so great...or so bad....or was it more because they simply happened to be there in a time of crisis?
Or is it a combination of both?
Well, yak away.
I am going to be a bit unconventional here (for a North American) and pick the Yellow Emperor, who ruled over China in its most prosperous and peaceful time. He is recognized by Taoists as the wisest, most beloved and successful ruler that China ever had, and the harmonious society he governed seems to have been a good testament to that.