This is quite vague. What period of history are you looking for? What regions of the world? There are many many different possible answers to this... from Emperor Meiji who lead to the modernization of Japan to Ali Bongo Ondimba, the current president of Gabon who has managed to both bring peace, self sustainability and industry to the nation but also take it out of an economic rut.

It is difficult to work with you saying "I want a leader who is not to mainstream" because that, literally, could turn up hundreds of results.

Why not try James K Polk ? A one term President of the United States, who was responsible for rounding out the territory of the 48 Continental States of the USA and pushing the frontier all the way to the Pacific Ocean. In Mexico, Polk is infamous, but sadly in the USA he is overshadowed by Abraham Lincoln. He also owned slaves, so he is considered controversial by modern standards. A good place to start is John Seigenthaler' book "James K Polk" which is part of the American Presidency Series, published back in 2004. Read that concise volume, then look at the other books published about this obscure president that focus on a topic that really interests you. Amazon carries several other books about James K Polk. Some focus on his imperialistic tendencies. Others look at his slave ownership. The bottom line is Polk is both obscure and infamous. He also left a lasting legacy, by settling the borders of the USA in the Pacific Northwest and the desert Southwest.