Who was the last leader of the Holy Roman Empire?
Holy Roman Emperor
The interregnum of the Holy Roman Empire is taken to have lasted from the deposition of Frederick II by Pope Innocent IV (1245, alternatively from the death of Frederick 1250 or the death of Conrad IV 1254) to the election of Rudolf I of Germany (1273).
Rudolf was not crowned emperor, nor were his successors Adolf and Albert.  
The next emperor was Henry VII, crowned on 29 June 1312 by Pope Clement V.