I dont think Portugal has the worst starting position. They can be attacked from north, east and south but they cant be attacked by west unless theres an amphibius invasion.

The HRE can be attacked on every direction and has powerful superpowers near it like France or the rich Italian Republics. Its still is the most difficult.

Second would be the Byzantines which have their main provinces separated by the bosphorous strait. Sounds like the Crusaders and Seljuk Turks could give you a bad time.