cortes-he and a few hundread adventurers took on an empire and won.
lenin-trotsky-a few hundread bolsheviks conquered the biggest country in the world
castro-he, and a few dozen men invaded cuba, and after a few years overthrew the government.
napoleon was not a conquerer. he never fought an aggressive war. all his wars were defensive.
hitler conquered poland, big deal, and france, got lucky. failed to conquer england because he started bombing london instead of the RAF, and totally screwed up operation barbarossa. if it were not for hitler, germany might have won the war, so hitler is not a good conqueror at all.
alexander the great might be #1. He took his army, and invaded a country 100x bigger then his own, and conquered it. within 7 years he increased the size of his empire 100 or 1000 fold.
genghis khan- okay, he created the biggest empire in history, but most of the land he conquered was empty wilderness.
it all depends on the criteria you use. most land conquered, hardest fought victory, greatest against-all-odds win, an empire that actually lasted, etc..