Traditionally history associates Charlemagne (Charles I) as the first Holy Roman Emperor. Pope Leo III crowned him emperor on Christmas day 800 AD. His reign and decisions about successors changes western history in many dynamic ways, creating the basis for the growing and development of one of the greatest and most powerful empires of all time.

The Holy Roman Empire as a Germanic state began under the rule of Otto I and many see this as the beginning of the Holy Roman Empire, Charlemagne and his successors maintain the title of Emperor until 924 AD, and then King Otto I would be crowned Emperor in 962 AD.

More Info: en.wikipedia.org